GIFT   OF 


SCIENCE 


FOR  THE 


SCHOOL   AND   FAMILY. 

PART  I. 
NATURAL  PHILOSOPHY. 

BY 

WORTHINGTON  HOOKER,  M.D., 

PBOFKSSOB  OF  THE  THEORY  AND  PRAOTIOE  OF  MEDICINE  IN  YALE  COLLEGE, 

AUTHOR  OF  "CHILD'S  HOOK  OF  NATURE,"  "FIUST  BOOK- IN  CHEMISTRY," 

"NATURAL  HISTORY,"  "CHEMISTRY,"  ETC. 

Kllustrateti  fc,»  Numerous  Hnjjrabfnjjs. 


SECOND    EDITION, 
REVISED   AND    ENLARGED. 


NEW  YORK: 
HARPER  &  BROTHERS,  PUBLISHERS 

FRANKLIN     SQUARE. 

1881. 


BY  DR.  WORTHINGTON  HOOKER. 


THE  CHILD'S  BOOK  OF  NATURE.  For  the  Use  of  Families  and  Schools; 
intended  to  aid  Mothers  and  Teachers  in  training  Children  in  the  Observa- 
tion of  Nature.  In  Three  Parts.  Illustrated  by  Engravings.  The  Three 
Parts  complete  in  one  volume.  Small  4to,  Cloth,  $1 12 ;  Separately,  Cloth, 
Part  I.,  45  cents ;  Parts  II.  and  III.,  48  cents  each. 

PART  I.  PLANTS. 

PART  II.  ANIMALS. 

PART  III.  AIR,  WATER,  HEAT,  LIGHT,  &c. 

FIRST  BOOK  IN  CHEMISTRY.  For  the  Use  of  Schools  and  Families.  Illus- 
trated by  Engravings.  Revised  Edition.  Square  4to,  Cloth,  48  cents. 

NA  TURA  L  HISTOR  Y.  For  the  Use  of  Schools  and  Families.  Illustrated  by 
nearly  300  Engravings.  12mo,  Cloth,  $1  00. 

SCIENCE  FOR  THE  SCHOOL  AND  FAMILY. 

PART  I.  NATURAL  PHILOSOPHY.    Illustrated  by  numerous  Engrav- 
ings.   Second  Edition,  Revised  and  Enlarged.     12mo,  Cloth,  $1  00. 

PART  II.  CHEMISTRY.    Illustrated  by  numerous  Engravings.    Second 
Edition,  Revised  and  Enlarged.     12mo,  Cloth,  $1  00. 

PART  III.  MINERALOGY  AND  GEOLOGY.    Illustrated  by  numerous 
Engravings.    12mo,  Cloth,  fl  00. 


Published  by  HARPER  &  BROTHERS,  Franklin  Square,  N.  Y. 


Any  of  the  abpiz_  works  seut  by  wail,  postage  prepaid,  to  any  part  of  the 
United  States  upon  receipt  of  the  price. 


Entered  according  to  Act  of  Congress,  in  the  year  18T8,  by  HARPKR  &  BROTHERS, 
in  the  Office  of  the  Librarian  of  Congress,  at  Washington. 


or. 


PREFACE. 


DANIEL  WEBSTER,  in  his  Autobiography,  speaks  thus  of  his  entering 
upon  the  study  of  law:  "I  was  put  to  study  in  the  old  way — that  is,  the 
hardest  books  first — and  lost  much  time.  I  read  Coke  on  Littleton  through 
without  understanding  a  quarter  of  it.  Happening  to  take  up  Espinasse's 
Law  of  Nisi  Prius,  I  found  I  could  understand  it ;  and  arguing  that  the 
object  of  reading  was  to  understand  what  was  written,  I  laid  down  the 
venerable  Coke  et  alios  similes  reverendos,  and  kept  company  for  a  time 
with  Mr.  Espinasse  and  others,  the  most  plain,  easy,  and  intelligible  writers. 
A  boy  of  twenty,  with  no  previous  knowledge  on  such  subjects,  cannot 
understand  Coke.  It  is  folly  to  set  him  on  such  an  author.  There  are 
propositions  in  Coke  so  abstract,  and  distinctions  so  nice,  and  doctrines 
embracing  so  many  conditions  and  qualifications,  that  it  requires  an  effort 
not  only  of  a  mature  mind,  but  of  a  mind  both  strong  and  mature,  to  un- 
derstand him.  Why  disgust  and  discourage  a  boy  by  telling  him  that  he 
')  nust  break  into  his  profession  through  such  a  wall  as  this  ?  I  really  often 
despaired.  I  thought  that  I  never  could  make  myself  a  lawyer,  and  was 
almost  going  back  to  the  business  of  school-keeping.  Mr.  Espinasse,  how- 
ever, helped  me  out  of  this  in  the  way  that  I  have  mentioned,  and  I  have 
always  felt  greatly  obliged  to  him." 

Here  is  most  graphically  depicted  a  defect  which  is  now,  as  it  was  then, 
very  prominent  in  all  departments  of  education.  It  is  even  more  so  in 

early  education  than  in  that  of  the  college  and  the  professional  school.    Even 

-•• 
in  tender  childhood  pupils  are  put  to  studying  books  of  which,  as  was  true 

of  Webster  with  his  Coke  on  Littleton,  they  do  not  understand  "a  quarter 
part."    If  the  rule  is  not  "the  hardest  books  first,"  there  are  many  things 

3G377'j 


viii  PREFACE. 

in  the  books  that  it  is  not  only  hard  but  impossible  for  them  to  understand. 
And  the  hardest  things  are  often  prft  first.  For  example,  in  a  very  popu- 
lar primary  geography  which  lies  before  me  the  pupil  is  introduced  to  the 
world  and  its  grand  divisions  at  the  outset,  while  he  is  taught  about  his 
own  state  and  country  only  at  the  conclusion  of  the  book.  And  this  un- 
natural mode  is  the  one  very  commonly  pursued.  Similar  criticism  can 
be  passed  upon  most  of  the  books  used  in  teaching  young  children.  Some 
of  them  are  wholly  useless.  This  is  true  of  the  grammars  for  primary 
schools.  The  formal  statements,  called  the  rules  of  grammar,  are  beyond 
the  understanding  of  very  young  scholars,  and  therefore  are  useless  bur- 
dens upon  their  memories.  They  are  as  useless  to  them  as  the  three 
fourths  of  Coke  which  Webster  could  not  understand  was  to  him. 

If  we  follow  education  upward  from  the  primary  school  we  find  the  same 
defect  throughout  the  whole  course.  In  the  books  which  are  used  in  teach- 
ing natural  science  it  is  especially  prominent.  Even  in  the  elementary 
books,  or  compendiums,  so  called,  formal  propositions  and  technical  terms 
render  the  study  uninviting,  and  to  a  great  extent  unintelligible.  The 
pupil  is  apt  to  be  disgusted  and  discouraged,  as  Webster  was  with  Coke  on 
Littleton,  and  for  the  same  reason. 

Another  defect  intimately  connected  with  that  of  which  I  have  spoken  is 
the  very  sparing  and  late  introduction  of  the  physical  sciences.  They  are 
generally  postponed  to  the  latter  part  of  the  course  of  education,  andrthen 
but  little  time  is  devoted  to  them.  Generally,  when  a  pupil  designs  to  go 
through  college,  the  study  of  these  sciences  is  wholly  neglected  in  his  prep- 
aration, because  a  knowledge  of  them  is  not  required  for  admission.  Then 
in  the  college  they  are  not  attended  to  till  the  latter  part  of  the  course,  and 
in  the  short  time  allotted  to  them  there  is  so  much  to  be  learned  that  the 
teaching  of  them  is  a  failure.  Especially  is  this  true  of  Chemistry  and 
Geology. 

This  defect  is  a  radical  one.  A  thorough  change  should  be  effected  in 
this  respect  in  the  whole  course  of  education.  The  natural  sciences  should 
be  made  prominent  from  the  beginning  to  the  end,  not  only  because  they 
are  of  practical  value,  but  also  because  they  are  as  useful  in  their  way  for 
mental  discipline  as  the  study  of  mathematics  and  of  language.  They  can 
be  taught  to  some  little  extent  to  the  youngest  pupils.  There  are  facts 


PREFACE.  ix 

about  air,  water,  and  the  various  objects  that  they  see  around  them,  which 
they  can  understand  if  they  be  presented  in  the  right  manner.  And  the 
busy  inquiries  which  they  make  after  the  reasons  of  the  facts,  and  their 
appreciation  of  them  if  stated  simply  and  without  technical  terms,  show 
the  appropriateness  of  such  teaching.  Children  are  really  very  good  phi- 
losophers in  their  way.  They  have  great  activity  not  only  of  their  percep- 
tive but  of  their  reasoning  faculties  also,  to  which  due  range  should  be 
given  in  their  education. 

Beginning  thus,  not  a  year  should  pass  during  the  whole  course  when 
the  pupil  shall  not  be  engaged  in  studying  some  one  of  the  physical  sciences 
to  some  extent.  This  continued  attention  to  such  studies  in  a  reason- 
able amount,  so  far  from  interfering  with  the  due  prosecution  of  the  other 
studies  deemed  so  essential,  will  so  promote  the  pupil's  advance  in  them  as  to 
more  than  make  up  for  the  time  that  is  taken  from  them.  It  will  do  this  not 
only  by  the  genial  influence  which  such  studies  exert  upon  the  mind,  but 
by  the  contributions  which  they  make  to  the  knowledge  of  language  and 
mathematics ;  for  language  is  largely  built  up  from  natural  objects  and  from 
the  acquisitions  of  science,  and  there  is  an  abundance  of  interesting  appli- 
cations of  portions  of  the  mathematics  in  the  facts  which  the  physical  sci- 
ences develop  to  us. 

I  have  said  that  the  teaching  of  the  natural  sciences  in  our  colleges  is 
generally  a  failure,  and  it  always  will  be  so  as  long  as  the  present  plan  is 
continued.  In  order  to  have  it  successful  there  must  be  the  same  gradation 
in  teaching  them  that  we  have  in  teaching  language  and  the  mathematics. 
The  college  student  needs  to  be  prepared  for  the  lectures  which  he  hears 
on  natural  philosophy,  chemistry,  etc.,  and  for  his  study  of  those  branches, 
by  previous  familiarity  with  the  simpler  portions  of  them  acquired  in  the 
school-room. 

There  is  another  very  important  reason  for  the  early  introduction  of  the 
physical  sciences  into  education.  By  far  the  larger  portion  of  pupils  in 
our  schools  stop  short  of  the  college,  or  even  the  academy  and  high-school. 
That  they  should  go  forth  into  the  world  with  no  knowledge  of  the  princi- 
ples that  lie  at  the  basis  of  the  arts  in  which  so  many  of  them  are  to  engage 
is  a  shame  and  a  wrong,  if  the  communication  of  such  knowledge  be  indeed 
practicable,  as  it  undoubtedly  is.  Even  those  who  are  not  to  engage  in 


X  PREFACE. 

these  arts  will  be  greatly  benefited  by  this  knowledge,  because  in  addition 
to  its  constant  practical  applications  in  the  management  of  life,  it  will  con- 
tribute to  their  mental  power,  and,  what  is  no  small  consideration,  to  their 
enjoyment ;  and  it  is,  in  fact,  requisite  to  constitute  them  well-informed 
persons. 

If  the  views  which  I  have  presented  be  correct,  there  should  be  a  series 
of  books  on  the  natural  sciences  carefully  adapted  to  the  different  periods 
of  the  course  of  study.  Those  intended  for  the  young  beginner  should  be 
exceedingly  simple,  and  should  not  attempt  to  present  anything  like  a  full 
view  of  the  subjects  treated.  They  should  deal  largely  with  familiar  facts 
or  phenomena.  The  terminology  of  science  and  formal  statements  of  prin- 
ciples, such  as  we  often  see  in  so-called  compendiums,  should  have  no  place 
in  them,  but  should  be  gradually  introduced  as  the  series  advances,  and 
should  be  made  complete  only  in  the  concluding  books. 

It  has  been  the  object  of  the  author  to  supply  a  part  of  such  a  series. 
The  first  book  in.  the  series  is  the  "Child's  Book  of  Common  Things," 
intended  to  teach  the  observation  of  familiar  facts,  or,  in  other  words,  the 
beginnings  of  philosophy,  to  children  as  soon  as  they  have  got  well  started 
in  reading.  Next  comes  the  "  Child's  Book  of  Nature,"  which  in  its  three 
parts  (Part  I.,  Plants;  Part  II.,  Animals;  Part  III.,  Air,  Water,  Light, 
Heat,  etc.)  extends  considerably  the  knowledge  of  the  philosophy  of  things 
which  the  child  has  obtained  from  the  first  book  in  the  series.  Then  follows 
the  "First  Book  in  Chemistry."  On  a  level  with  this  is  my  "First  Book 
in  Physiology."  The  next  step  in  the  gradation  brings  us  to  three  books 
under  one  title,  "Science  for  the  School  and  the  Family;"  Part  I.,  Nat- 
ural Philosophy;  Part  II.,  Chemistry  ;  Part  III.,  Mineralogy  and  Geology. 
On  a  level  with  these  is  another  book,  "Natural  History,"  and  on  another 
still  is  to  be  written  an  "Introduction  to  Botany." 

The  three  books,  of  which  the  present  is  one,  are  intended  for  the  older 
scholars  in  what  are  commonly  called  grammar-schools.  At  the  same  time 
they  are  suited  to  scholars  who  are  advanced  to  a  higher  grade  who  have 
not  gone  through  the  previous  books  of  the  series.  The  preparation  of 
books  especially  adapted  to  high-schools  and  colleges  I  have  left  to  others, 
except  in  one  branch  of  science,  Physiology,  on  which  I  some  years  ago 
published  a  work  entitled  "Human  Physiology." 


PREFACE.  XI 

All  of  these  books  are  from  the  press  of  Harper  &  Brothers  except  the 
two  works  on  Physiology,  published  by  Sheldon  &  Co.,  New  York,  and  the 
"  Child's  Book  of  Common  Things,"  published  by  Peck,  White,  &  Peck, 
New  Haven. 

The  general  plan  and  style  of  these  books  are  very  different  from  what 
we  see  in  most  of  the  books  for  schools  on  the  same  subjects.  The  order 
of  the  subjects  and  the  mode  of  developing  them  differ  from  the  stereotype 
plan  which  has  so  generally  been  adopted.  One  prominent  feature  is  the 
free  use  of  illustrations  from  familiar  phenomena.  This  leads  the  pupil  to 
reason  or  philosophize  about  common  things,  thus  giving  an  eminently  prac- 
tical character  to  his  knowledge.  At  the  same  time  it  makes  the  books 
suitable  for  use  in  the  family  as  well  as  the  school,  between  which  there 
should  be  more  common  ground  than  the  present  mode  of  education  allows. 

The  style  which  I  have  chosen  for  all  the  books  I  have  written  for  use 
in  teaching  is  what  may  be  called  the  lecture  style.  There  are  three  other 
kinds  of  style  which  are  more  commonly  used  in  school-books.  The  most 
common  is  what  I  term  the  formal  statement  style.  In  this  principles  and 
rules  are  stated,  and  then  illustrations  are  given.  This  makes  a  formal  and 
uninviting  book.  The  bare  skeleton  of  the  science  is  generally  for  the  most 
part  presented,  and  the  young  pupil  is  apt  to  learn  the  statements  by  rota 
without  understanding  them.  It  is  a  style  fitted  only  for  books  intended 
for  advanced  scholars.  Another  style  is  the  catechetical.  This  is  an  un- 
natural mode  of  communicating  knowledge ;  and,  besides,  it  encourage* 
learning  by  rote  as  the  formal  statement  style  does.  In  the  third  style,  th< 
dramatic,  conversations  are  held  between  the  teacher  and  some  learners. 
The  chief  objection  to  this  is  that  it  undertakes  to  put  in  permanent  shape 
what  should  be  extemporized  in  the  recitation.*  What  is  needed  in  the 
book  is  simply  clear  and  concise  statement  in  an  interesting  style,  and  the 
living  teacher  and  his  scholars  can  best  furnish  the  conversational  element 
as  the  recitation  goes  on. 

In  the"  lecture  style  there  may  be  and  should  be  as  much  precision  of 
statement  as  in  the  formal  statement  style,  while  it  is  more  interesting, 
because  it  is  the  natural  mode  of  communicating  knowledge.  In  this  style 
the  facts  are  ordinarily  so  stated  as  to  develop  principles ;  while  in  the  other 
the  order  is  reversed,  the  principles  being  first  stated  and  the  facts  given 


X1L  PREFACE. 

afterwards.  One  of  the  most  successful  books  ever  used  in  our  colleges — 
"Paley's  Natural  Theology" — is  in  the  lecture  style,  and  it  is  a  matter  of 
surprise  that  this  fact  has  had  so  little  influence  with  those  who  have  pre- 
pared books  for  instruction. 

Whatever  may.  be  true  of  advanced  scholars,  in  teaching  the  youny  stu- 
dent in  science  bare,  dry  statement  should  be  avoided,  and  the  subjects 
should  be  presented  in  all  their  attractive  features.  I  would  not  be  under- 
stood as  advocating  the  dressing  up  of  science  in  adventitious  charms. 
This  is  not  necessary.  Science  possesses  in  itself  an  abundance  of  charms, 
which  need  only  to  be  properly  developed  to  attract  the  young  mind ;  and 
the  lecture  style  furnishes  the  best  vehicle  for  such  a  development. 

One  grand  essential  for  giving  interest  to  any  study  is  the  presentation 
of  the  various  points  in  the  natural  order  in  which  they  should  enter  the 
mind.  They  should  be  so  presented  that  each  portion  of  a  look  shall  make 
the  following  portions  more  interesting  and  more  easily  understood.  This 
principle,  which  is  so  commonly  transgressed,  I  have  endeavored  to  observe 
strictly  in  the  preparation  of  these  volumes. 

Questions  are  inserted  for  those  teachers  who  desire  to  use  them.  There 
is  also  an  Index. 

W.  HOOKER. 

January,  18S8. 


PREFACE  TO  THE  SECOND  EDITION. 


IN  revising  this  work,  its  essential  features,  as  fully  explained  in  the 
Author's  Preface,  have  been  carefully  preserved;  at  the  same  time,  many 
portions  have  been  entirely  rewritten  and  much  new  matter  has  been 
added.  The  chapter  on  Galvanism,  omitted  in  the  Second  Edition  of 
Part  II.,  Chemistry,  has  been  revised  and  inserted  in  the  present  volume. 
Many  new  wood-cuts  have  been  introduced,  taken  chiefly  from  German 
sources. 

In  the  First  Edition  of  this  work  the  author  makes  frequent  reference  to 
Dr.  Neil  Arnott's  Elements  of  Physics;  the  editor  acknowledges  his  in- 
debtedness to  the  Seventh  Edition  of  the  same  for  many  of  the  illustra- 
tions of  physical  phenomena,  and  for  suggesting  the  arrangement  of  matter 
in  certain  parts  of  this  revision. 

II.  CARRINGTON  BOLTON,  Ph.D. 

TRINITY  COLLEGE,) 
Nay,  1873.        J 


A2 


CONTENTS. 


CHAPTER  FAGB 

I.   MATTER 17 

II.   PROPERTIES   OF  MATTER 28 

III.  PROPERTIES   OF   MATTER   (CONTINUED) 39 

IV.  ATTRACTIONS   OF  MATTER 51 

V.   ATTRACTIONS   OF   MATTER    (CONTINUED) 67 

VI.   CENTRE   OF   GRAVITY 79 

VII.   MOTIONS   OF   MATTER 93 

VIII.   MOTIONS    OF  MATTER  (CONTINUED) 112 

IX.   THE   SIMPLE  MACHINES 133 

X.   HYDROSTATICS 159 

XI.   SPECIFIC    GRAVITY 181 

XII.   HYDRAULICS 196 

XIII.  PNEUMATICS 208 

XIV.  SOUND 235 

XV.   HEAT 252 

XVI.  HEAT  (CONTINUED) 281 

XVII.   LIGHT 312 

XVIII.   ELECTRICITY 351 

XIX.   GALVANISM 375 

XX.   MAGNETISM 397 

APPENDIX. — THE  METRIC  SYSTEM 415 

INDEX...                                                                                      ..  423 


NATURAL    PHILOSOPHY. 


CHAPTER  I. 

MATTER. 

1.  Introduction. — Since  you  are  about  to  begin  the  study 
of  the  series  of  books  embraced  under  the  general  title 
"Science  for  the  School  and  Family,"  and  of  which  this 
work  on  Natural  Philosophy  forms  the  first  volume,  it  is 
necessary  that  you  should  clearly  understand  what  is  meant 
by  Science.  This  word  literally  means  knowledge;  but  in 
the  sense  in  which  it  is  commonly  employed,  science  de- 
notes a  systematic  and  orderly  arrangement  of  knowledge. 
Superficial  and  incomplete  information  on  any  given  topic, 
interwoven  with  fictions  of  the  imagination,  does  not  con- 
stitute science ;  on  the  contrary,  science  implies  a  profound, 
penetrating,  and  comprehensive  knowledge  based  on  gen- 
eral truths  and  fundamental  principles.  Since  the  human 
mind  is  capable  of  apprehending  the  phenomena  of  the 
whole  universe  of  nature  and  of  thought,  and  can  subject 
them  to  the  action  of  both  the  reason  and  the  imagination, 
it  is  evident  that  science  in  its  fullest  signification  is  well- 
nigh  boundjess  in  its  range  and  infinite  in  the  variety  of  its 
material.  To  facilitate  the  study  of  so  vast  a  subject  as 
the  sum  of  human  knowledge,  we  naturally  resort  to  sys- 


18  "*  •-**"      NAtUBA.L 

tematic  divisions  and  methods  of  classification.  To  enter 
upon  an  examination  of  the  various  systems  of  classifying 
the  arts  and  sciences  proposed  by  different  authorities  is 
quite  foreign  to  our  object,  and  we  shall  adopt  without 
argumentation  the  views  of  the  late  Dugald  Stewart,  for- 
merly Professor  of  Moral  Philosophy  in  the  University  of 
Edinburgh.  After  carefully  criticising  the  schemes  of  his 
predecessors,  Professor  Stewart  concludes  that  the  two 
most  general  heads  on  which  to  found  an  encyclopedical 
classification  of  science  are  MIND  and  MATTER.  "No 
branch  of  human  knowledge — no  work  of  human  skill — can 
be  mentioned  which  does  not  obviously  fall  under  the  for- 
mer head  or  the  latter." 

The  sciences  of  mind  and  of  matter  are  susceptible  of 
subdivisions ;  the  former  embraces  Pure  Mathematics,  Met- 
aphysics, Mental  and  Moral  Philosophy,  Political  Economy, 
Sociology,  and  other  subjects  upon  which  we  do  not  dwell; 
the  latter  deals  with  the  less  abstract  topics  included  in 
the  sciences  of  Natural  History,  Astronomy,  Chemistry, 
and  Natural  Philosophy  or  Physics. 

Natural  History  includes  within  its  extended  limits  the 
sciences  of  minerals,  or  Mineralogy,  and  of  rocks,  or  Geol- 
ogy; of  plants,  or  Botany;  of  animals,  or  Zoology;  and 
of  man,  or  Anthropology.  Under  this  head,  too,  may  be 
placed  Medical  Science,  Physiology,  etc.  Astronomy,  as 
you  know,  teaches  about  the  heavenly  bodies,  their  motions, 
magnitudes,  and  periods  of  revolution  ;  Chemistry  deals 
with  the  internal  composition  of  substances  and  their 
mutual  reactions ;  and  Natural  Philosophy,  or  Physics,  may 
be  defined  as  that  branch  of  science  which  treats  of  the 
properties  and  laws  of  matter.  The  term  Natural  Philoso- 
phy itself  embraces  a  whole  series  of  sciences,- as  will  ap- 
pear from  an  examination  of  the  following  chapters. 

2.  Effects  of  Matter  on  the  Senses. — The  substance  of 


MATTEE.  1 9 

which  material  objects  are  made  is  called  matter.  Matter 
is  often  defined  as  anything  perceptible  by  the  senses— a 
statement  which  demands  closer  consideration.  Some  forms 
of  matter  can  be  perceived  by  all  the  senses ;  others  can  be 
perceived  by  only  a  part  of  them ;  some  by  only  one.  Air 
you  cannot  see,  nor  smell,  nor  taste ;  but  you  can  feel  it, 
and  hear  and  see  the  effects  of  its  motion.  Sometimes  mat- 
ter affects  only  the  sense  of  smell,  or  that  with  the  sense  of 
taste.  Sea-air  smells  of  salt ;  but  the  salt  in  the  air  is  so 
finely  divided  we  cannot  see  it.  And  yet  it  is  the  salt, 
entering  the  nostrils  and  coming  in  contact  with  the  sen- 
sitive fibres  of  the  nerves  of  smell,  that  produces  the  effect. 
Thus  when  we  smell  a  flower,  matter  comes  from  it  in  par- 
ticles so  minute  that  no  microscope  can  detect  them,  but 
they  produce  sensation  when  they  strike  upon  the  nerve. 

Diflerent  kinds  of  matter  constitute  the  substances  of 
which  bodies  are  made.  These  bodies  are  subject  to  change 
of  state,  form,  and  mutual  relation ;  and  a  study  of  these 
phenomena  is  the  province  of  Natural  Philosophy. 

3.  Forms  of  Matter.  —  Matter  appears  in  three  forms: 
solid,  liquid,  and  gaseous  or  aeriform  —  that  is,  like  air. 
Sometimes  matter  is  spoken  of  as  having  only  two  forms — 
solid  and  fluid.  In  this  case  fluids  are  divided  into  two 
classes,  the  elastic  and  non-elastic.  The  air  and  the  vari- 
ous gases  and  vapors  are  the  elastic  fluids;  while  those 
which  are  called  liquids  are  the  non-elastic  fluids.  A  foot- 
ball bounds  because  the  air  in  it  is  an  elastic  fluid.  If  it 
were  filled  with  a  non-elastic  fluid,  as  water,  it  would  not 
bound.  When  water  takes  the  form  of  steam  it  becomes 
elastic.  Though  it  was  formerly  very  common  to  use  the 
expression  elastic  fluids,  the  division  of  matter  into  three 
forms  is  the  one  now  usually  recognized,  liquids  having 
been  found  to  be  feebly  elastic. 

Solids. — In  solid  matter  the  particles  cannot  be  moved 


20  NATURAL  PHILOSOPHY. 

about  among  each  other;  but  each  particle  generally  re- 
tains the  same  position  in  relation  to  those  particles  which 
are  around  it — in  other  words,  it  does  not  change  its  neigh- 
borhood. This  is  more  true  of  some  solids  than  of  others. 
It  is  absolutely  true  of  such  hard  solids  as  granite  and  the 
diamond.  In  these  the  particles  always  maintain  the  same 
relative  position.  But  it  is  not  so  with  gold  or  lead.  By 
hammering  these  you  can  change  greatly  the  relative  posi- 
tion of  their  particles.  India-rubber  is  a  solid,  but  the 
relative  position  of  its  particles  can  be  much  altered  in 
various  ways. 

Liquids. — It  is  characteristic  of  a  liquid  that  its  particles 
change  their  relative  position  from  the  slightest  causes. 
It  is  in  strong  contrast  with  solids  in  this  respect.  When 
you  move  any  portion  of  a  solid  body  you  move  all  the 
other  portions  of  it,  and  generally  in  the  same  direction. 
But  a  body  of  liquid  cannot  be  moved  altogether  as  one 
body  except  by  confining  it,  as,  for  example,  in  the  case  of 
a  water-pipe  or  a  syringe.  And  then,  the  moment  that  the 
water  can  escape,  the  particles  use  their  liberty  of  altering 
their  relative  position.  Since  wind  and  other  agents  act 
continually  upon  water,  no  particle  stays  for  any  length  of 
time  in  the  neighborhood  of  the  same  particles.  "Unstable 
as  water"  is,  then,  an  exceedingly  significant  expression. 
Water  is  never  at  rest.  A  particle  of  it  may  at  one  time 
be  floating  on  the  surface  of  the  ocean,  and  at  another  be 
in  depths  beyond  the  reach  of  man.  It  flies  on  the  wings 
of  the  wind,  falls  in  the  rain,  runs  in  the  stream,  is  exhaled 
from  a  leaf,  trembles  in  the  dew-drop,  flows  in  the  blood  of 
•an  animal  or  in  the  sap  of  a  plant,  and  is  always  hurrying 
along  in  its  ever-changing  course. 

Gases. — The  particles  of  gaseous  or  aeriform  substances 
move  among  each  other  even  more  freely  than  those  of  a 
liquid.  Air,  therefore,  is  more  unstable  and  restless  than 


MATTER.  21 

water.  Even  when  the  air  seems  to  be  perfectly  still  its 
particles  are  moving  about  among  each  other.  You  can 
see  this  to  be  true  if  you  darken  a  room,  leaving  a  single 
shutter  a  little  open.*  Where  the  light  enters  you  will  see 
motes  flying  about  in  every  direction,  which  would  not  be 
the  case  if  the  air  were  really  at  rest.  The  particles  of  air 
have  a  greater  range  of  travel  than  those  of  water;  for  the 
sea  of  atmosphere  which  envelopes  the  earth  rises  to  the 
height  of  about  fifty  miles.  How  far  water  rises  by  evap- 
oration we  know  not ;  but  it  is  not  at  all  probable  that  it 
rises  to  the  uppermost  regions  of  the  atmosphere. 

f  4.  Pilling  of  Spaces  by  Liquids  and  Gases. — It  is  the  free- 
ness  with  which  the  particles  of  liquids  and  gases  move  among  each  other 
that  enables  them  to  insinuate  themselves  into  spaces  everywhere.  They 
are  ever  ready  to  enter  into  any  substances  which  have  interstices  or  pores 
of  such  size  as  will  admit  them.  Mingled  with  the  grains  of  the  soil  are 
not  only  water,  but  air  and  gases.  These  are  present  also  in  all  living 
substances,  both  vegetable  and  animal.  Water  forms  the  chief  part  of  sap 
and  of  blood,  and  water  is  always  accompanied  by  air  and  other  gases. 
Part  of  the  air  we  inhale  enters  the  blood  in  the  lungs,  and  courses  with  it 
through  the  system.  The  fishes  could  not  live  in  water  if  no  air  were 
mingled  with  it.  This  can  be  proved  by  experiment.  If  you  put  a  fish 
into  a  close  vessel  it  will  soon  die,  because  it  uses  up  all  the  air  held  in 
solution  by  the  water.  In  an  open  vessel  the  fish  is  kept  alive  by  the  con- 
stant accessions  of  fresh  air  to  the  water.  Advantage  is  taken  of  this  in 
the  preservation  of  fish  in  large  aquaria,  where  air  is  constantly  pumped 
into  the  water  contained  in  the  tanks. 

Solution. — When  solid  substances  dissolve  in* water  or  other  liquids,  the 
particles  of  the  solids  penetrate  between  the  little  particles  of  the  liquid, 
and  it  is  owing  to  the  freedom  with  which  the  particles  of  water  move 
about  among  each  other  that  they  are  able  to  take  in  among  them  the 
minute  particles  of  the  solid.  §  12. 

5.  Relation  of  Heat  to  the  Forms  of  Matter. — Some  kinds 
of  matter  are  seen  in  all  the  three  forms.  Whether  these 
shall  assume  one  form  or  another  depends  on  the  amount 
of  heat  present.  Thus  when  water  is  solid,  ice,  it  is  be- 


22  NATURAL   PHILOSOPHY. 

cause  a  part  of  its  heat  is  gone.  Apply  heat,  and  it  becomes 
a  liquid,  water.  Increase  the  heat  to  the  boiling  point,  and 
it  becomes  steam,  or  an  aeriform  substance.  Alcohol  has 
only  two  forms — liquid  and  aeriform^  It  has  never  been 
frozen.  Iron  is  usually  solid ;  but  in  the  foundry,  by  the 
application  of  great  heat,  it  is  liquefied.  Mercury  is  liquid 
in  all  ordinary  temperatures;  but  it  often  becomes  solid  in 
the  extreme  cold  of  arctic  winters.  A  mercurial  thermom- 
eter is  of  course  useless  under  such  circumstances,  and  the 
alcoholic  thermometer  is  relied  upon  to  denote  the  degree 
of  cold.  The  difference  between  mercury,  water,  and  iron 
in  regard  to  the  liquid  state  is  this :  Comparatively  little 
heat  is  required  to  make  mercury  liquid,  while  more  is  re- 
quired for  this  condition  in  water,  and  much  more  in  the 
case  of  iron. 

6.  The  Nature  of  Matter  Unknown. — Let  us  now  inquire, 
what  do  we  know  of  the  nature  of  matter  ?  Can  we  say 
that  we  know  anything  of  it  ?  We  may  observe  its  phe- 
nomena and  learn  its  properties ;  but,  with  our  most  search- 
ing analyses,  we  can  no  more  determine  the  nature  of  mat- 
ter than  we  can  that  of  spirit.  Newton  supposed  "  that 
God  in  the  beginning  formed  matter  in  solid,  massy,  hard, 
impenetrable  particles."  This  he  believed  to  be  true  of 
liquids,  and  even  of  gases,  as  well  as  of  solids.  In  the  gas 
these  hard  particles  are  much  farther  apart  than  in  the 
solid.  The  supposition  is  a  very  probable  one ;  but  if  it 
be  true,  it  does  not  teach  us  what  matter  is,  for  it  leaves 
us  in  the  dark  as  to  the  nature  of  the  particles.  Newton 
further  supposed  that  these  particles  have  always  remained 
unaltered  amid  all  the  changes  that  are  taking  place ;  these 
changes  being  occasioned  by  "  the  various  separations  and 
new  associations  and  motions  of  these  permanent  particles." 
When,  for  example,  anything  is  burned  up,  as  it  is  ex- 
pressed, not  one  of  the  particles  is  destroyed;  they  merely 


MATTER.  23 

assume  new  forms.  Though  most  of  the  substance  has 
flown  off  in  the  form  of  gas,  the  ultimate  particles  compos- 
ing the  gas  are  the  same  as  when  they  made  a  part  of  the 
solid  substance ;  and  they  may  soon  again  become  a  part 
of  some  new  solids.  Such  changes  in  the  forms  of  matter 
are  everywhere  going  on ;  and  when  you  study  Chemistry, 
Part  II.  of  this  Series,  you  will  become  familiar  with  them. 
7.  The  Constitution  of  Matter. — The  nature  of  the  inter- 
nal structure  of  matter  cannot  be  experimentally  deter- 
mined, and  we  are  again  obliged  to  resort  to  hypotheses 
or  suppositions.  Two  hypotheses  of  the  internal  constitu- 
tion of  matter  have  been  proposed ;  according  to  the  first 
matter  is  homogeneous  throughout  its  mass,  and  pre- 
sents no  interior  void,  or,  in  other  words,  is  continuous. 
This  is  the  supposition  of  Descartes,  an  eminent  French 
philosopher  of  the  seventeenth  century,  but  possesses  so 
small  a  degree  of  probability  that  the  scientific  world  has 
abandoned  it  for  the  second  hypothesis.  According  to  this, 
bodies  consist  of  an  agglomeration  of  an  immense  number 
of  excessively  small  particles  called  molecules  /  these  small 
particles  do  not  touch  each  other,  but  are  held  in  their  places 
by  reciprocal  attraction ;  they  are  supposed  to  be  continu- 
ally in  motion,  the  amplitude  of  their  oscillations  varying 
.with  the  form  of  the  body,  solid,  liquid,  or  gaseous.  This 
hypothesis  originated  with  ancient  Greek  philosophers  about 
twenty-two  centuries  ago,  but  has  been  modified  consider- 
ably in  modern  times.  There  are  many  reasons  for  accept- 
ing this  view,  some  of  which  we  will  state  briefly.  In  the 
first  place,  matter  is  divisible,  as  will  be  explained  more 
fully  in  Chapter  II,  and  it  is  difficult  to  comprehend  its 
divisibility  if  it  contains  no  void  spaces.  The  solubility  of 
solids  is  explained  in  §  4  by  a  reference  to  this  hypoth- 
esis. Secondly,  the  expansion  of  bodies  by  heat,  and  their 
contraction  on  cooling,  is  readily  explained  by  the  molecular 


24  NATURAL   PHILOSOPHY. 

hypothesis,  for  we  may  conceive  that  the  spaces  which 
separate  the  molecules  become  larger  or  smaller  in  conse- 
quence of  the  separation  or  approach  of  the  latter.  The 
fact  that  bodies  assume  three  states,  solid,  liquid,  and  gase- 
ous, is  explained  in  a  somewhat  similar  manner.  Thirdly, 
when  two  substances  are  brought  together  it  often  happens 
that  they  intimately  interpenetrate,  and,  each  losing  its 
characteristic  property,  they  acquire  new  properties  com- 
mon to  both.  Chemical  science  affords  us  numerous  exam- 
ples of  such  combinations.  Sodium,  for  instance,  is  a  white 
lustrous  metallic  substance,  as  soft  as  wax,  fusible  at  a  low 
temperature,  lighter  than  water,  tarnishing  very  readily, 
and  decomposing  water  at  ordinary  temperatures ;  chlorine, 
on  the  other  hand,  is  a  'yellowish-green  gas,  heavier  than 
air,  of  disagreeable  odor,  fatal  to  animals  when  breathed 
by  them,  possessing  strong  bleaching  power,  and  soluble 
in  water;  and  these  two  strangely  dissimilar  substances 
combine  to  form  the  white  crystalline  solid,  common  salt, 
so  indispensable  to  man  and  animals. 

Such  facts  as  these  are  comprehensible  if  we  adopt  the 
view  that  matter  is  composed  of  exceedingly  small  par- 
ticles, but  it  is  impossible  to  explain  them  on  the  hypoth- 
esis of  Descartes.  Taking  the  example  given,  sodium  and 
chlorine  are  each  regarded  as  made  up  of  minute  particles 
having  the  properties  named,  and  when  combination  takes 
place  between  individual  particles  they  are  associated  in 
new  forms  and  recognized  as  common  salt. 

Fourthly,  the  various  phenomena  of  light  and  of  heat, 
and  certain  chemical  laws  which  will  be  explained  in  Part 
II.,  combine  with  the  foregoing  proofs  to  form  an  argu- 
ment in  favor  of  the  molecular  hypothesis  not  easily  out- 
weighed. 

8.  Molecules. — These  particles  of  matter  are  so  minute 
that  they  have  never  been  seen  by  man ;  the  smallest  par- 


MATTER.  25 

ticle  visible  with  the  most  perfect  microscope  is  probably 
greater  than  ^oVo  °f  a  millimetre*  in  diameter,  and  it  bus 
been  calculated  that  to  see  these  molecules  we  should  re- 
quire a  lens  magnifying  from  500  to  2000  times  greater 
than  any  we  now  possess.  We  are  about  as  far  from  see- 
ing the  largest  molecules  as  we  should  be  from  reading 
with  the  unaided  eye  the  letters  on  a  page  of  this  book  at 
the  distance  of  one  third  of  a  mile. 

Eminent  philosophers  have. estimated  the  size  of  molecules,  and  have  ob- 
tained remarkable  coincidence  of  results  from  independent  and  widely 
different  data ;  from  these  calculations  it  may  be  concluded  with  a  high 
degree  of  probability  that  in  ordinary  liquids  or  solids  the  diameter  of  the 
molecule  is  less  than  the  ten-millionth  and  greater  than  the  two-hundred- 
millionth  of  a  millimetre. 

Molecules  are  believed  to  be  continually  in  motion,  and 
with  very  great  velocity,  estimated  to  average  seventeen 
miles  a  minute.  This  motion  is  in  all  directions,  and  of  an 
oscillatory  character,  the  particles  flying  to  and  fro  through 
excessively  small  paths,  the  diameter  of  which  varies  with 
the  nature  of  the  substance. 

Not  only  do  molecules  vary  in  size  and  velocity  of  mo- 
tion, but  they  also  differ  among  themselves  in  weight; 
their  weight  depends  on  that  of  the  atoms  (see  next  para- 
graph) composing  them,  and  can  be  determined  in  accord- 
ance with  laws  explained  in  Part  II.,  Chemistry. 

Atoms. — Molecules  are  believed  to  he  composed  of  still 
smaller  particles  of  matter  called  atoms.  The  number  of 
atoms  forming  a  molecule  varies  greatly;  in  certain  cases 
a  molecule  contains  but  one  atom,  in  others  several  hun- 
dred. If  the  atoms  composing  the  molecule  are  of  one  and 
the  same  substance,  the  molecule  is  said  to  be  simple  or 
elementary ;  if  atoms  of  diverse  kinds  of  matter  unite  to 

*  See  Appendix,  Metric  System  of  Weights  and  Measures. 


26  NATURAL   PHILOSOPHY. 

form  a  molecule,  it  is  said  to  be  compound.  Thus  the  mole- 
cules of  copper  are  made  up  of  atoms  of  copper  solely,  and 
copper  is  consequently  regarded  as  an  elementary  body ; 
on  the  other  hand,  molecules  of  sugar,  of  saltpetre,  etc.,  are 
compound,  the  first  named  being  made  up  of  twelve  atoms 
of  carbon,  twenty-two  of  hydrogen,  and  eleven  of  oxygen, 
and  the  second  containing  one  atom  of  potassium,  one  atom 
of  nitrogen,  and  three  of  oxygen. 

Atoms  of  different  kinds  of  matter  vary  in  weight,  those 
of  gold  and  lead,  for  example,  being  much  heavier  than 
those  of  hydrogen. 

By  taking  advantage  of  the  extraordinary  tinctorial  power  of  certain 
aniline  dyes,  experiments  have  been  made  which  show  that  an  atom  of 
hydrogen  undoubtedly  weighs  less  than  0.000,000,000,054  gramme ;  ac- 
cording to  another  authority  the  weight  of  a  hydrogen  atom  cannot  be  less 
than  0.000,000,000,000,000,000,000,000,0075  of  a  gramme.  It  is  im- 
possible to  conceive  of  such  minute  quantities,  nor  have  these  figures  any 
practical  value ;  we  give  them  chiefly  as  a  subject  of  curiosity. 

The  smallest  particle  of  matter  which  can  exist  in  a  free 
state  is  the  molecule ;  atoms  exist  in  combination  only,  and 
are  sometimes  defined  as  the  smallest  particle  of  matter 
which  can  enter  into  the  composition  of  a  molecule. 

The  physical  properties  of  bodies,  hardness,  transparency, 
elasticity,  etc.,  depend  mainly  on  their  molecular  relations; 
the  chemical  properties  on  their  atomic  relations.  The 
study  of  the  atomic  composition  of  bodies,  and  of  the  laws 
governing  their  combination,  is  the  province  of  chemistry, 
and  will  be  fully  explained  in  Part  II. 

9.  Matter  acted  upon  by  Forces. — The  constant  changes 
of  form  and  mutual  relations  of  bodies  are  caused  by  ex- 
ternal agents  called  forces.  Gravitation,  heat,  light,  and 
electricity  are  some  of  these  forces.  These  physical  forces 
were  formerly  regarded  as  exceedingly  attenuated  forms 
of  matter,  and,  since  they  have  no  weight,  were  called  ini- 


MATTER.  27 

ponderable  agents.  At  present,  however,  the  opinion  pre- 
vails that  the  phenomena  caused  by  these  forces  result  from 
the  motions  of  the  inappreciably  minute  particles  of  matter. 
It  is  believed  also  that  there  is  in  reality  but  one  force  in 
nature,  and  that  heat,  light,  and  electricity  are  different 
manifestations  of  this  force.  Just  as  a  certain  amount  of 
matter  was  created,  and  continues  to  exist  without  diminu- 
tion in  quantity,  in  like  manner  a  definite  amount  of  force 
was  created,  and  this,  too,  is  indestructible,  though  mani- 
festing itself  in  various  ways  to  our  senses.  It  will  be 
shown  farther  on  that  the  forces  of  heat,  light,  and  electric- 
ity are  mutually  convertible  and  equivalent.  They  are  all 
referred  back  to  a  common  origin — motion  of  the  molecules 
of  matter. 

The  agency  of  the  physical  forces  is  of  great  importance 
and  is  very  active,  producing  constant  changes  throughout 
nature ;  they  are  also  obviously  and  immediately  essential 
to  life.  

QUESTIONS.* 

[The  numbers  refer  to  the  sections.] 

1.  What  is  meant  by  Science  ?  What  is  said  about  the  classification  of 
knowledge?  Name  some  of  the  chief  subdivisions. — 2.  What  is  said  of 
,he  effects  of  matter  on  the  senses  ? — 3.  What  are  the  forms  of  matter  ? 
Illustrate  the  difference  between  elastic  and  non-elastic  fluids.  What  is 
said  of  the  union  of  the  particles  of  a  solid?  Give  the  difference  noted 


*  Teachers  differ  much  in  their  plans  of  conducting  recitations.  Some  are  very 
minute  in  their  questions ;  while  others  go  to  the  other  extreme,  and  merely  name 
the  topics,  the  pupils  being  expected  to  give  in  full  what  is  eaid  upon  them.  Nei- 
ther of  these  plans  should  be  adopted  exclusively,  but  the  mode  of  recitation  should 
be  much  varied  from  time  to  time.  This  variety  is  somewhat  aimed  at  in  the  ques- 
tions which  we  have  prepared,  though  in  no  case  are  the  questions  as  minute  as  they 
should  occasionally  be  made  by  the  teacher.  It  would  be  well  to  have  the  pupils 
draw  many  of  the  figures  upon  the  blackboard,  and  then  recite  from  them.  By 
drawing  the  simplest  figures  first  sufficient  skill  may  be  acquired  to  enable  the  pupil 
to  draw  those  which  are  quite  difficult. 


28  NATURAL   PHILOSOPHY. 

between  different  solids.  How  does  a  liquid  differ  from  a  solid  ?  Give  in 
detail  what  is  said  of  water.  What  is  said  of  the  particles  of  gaseous  sub- 
stances ?  What  of  the  atmosphere  ?  What  of  the  vapor  in  it  ?-^4.  What 
is  said  of  the  entering  of  liquids  and  gases  into  interstices  ?  What  of  the 
mingling  of  gases  with  liquids  ?  Give  the  illustration  in  regard  to  fishes. 
What  is  said  of  the  solution  of  solids  in  liquids  ?  What  of  the  evaporation 
of  water  in  the  air  ? — 5.  Illustrate  the  influence  of  heat  on  the  forms  of 
matter.  What  is  said  of  the  thermometer  ?  What  of  mercury,  water,  and 
iron  in  relation  to  the  liquid  state  ? — 6.  What  is  said  of  our  knowledge  of 
matter  ?  What  was  the  supposition  of  Newton  about  the  composition  of 
matter  ?  What  is  said  of  the  changes  of  matter  ? — 7.  What  two  supposi- 
tions have  been  made  as  to  the  internal  structure  of  matter  ?  AVith  whom 
did  the  more  probable  one  originate?  Name  the  principal  reasons  for 
adopting  the  molecular  hypothesis  ?  Describe  in  full  the  chemical  reasons. 
— 8.  What  is  said  of  molecules  ?  How  near  are  we  to  seeing  them  ?  How 
large  are  they  believed  to  be  ?  What  of  their  motions  ?  Of  what  are  mole- 
cules composed?  When  are  they  elementary?  Give  examples.  When 
are  they  said  to  be  compound  ?  Give  examples.  What  is  said  of  the 
weight  of  atoms  of  hydrogen  ?  What  properties  of  matter  depend  mainly 
on  the  molecular  relations  of  bodies? — 9.  What  forces  act  upon  matter? 
What  is  said  about  the  unity  of  force  ? 


CHAPTER  II. 

PROPERTIES    OF   MATTER. 

10.  Properties  of  Matter. — All  matter  has  properties  or 
qualities ;  these  differ  in  the  different  kinds  of  matter  as 
well  as  in  substances  of  the  same  class.  Some  of  these 
properties  are  common  to  all  kinds  and  forms  of  matter, 
and  are  called  universal  properties  ;  others  are  peculiar  to 
certain  substances  and  are  called  specific  properties.  The 
properties  of  matter  which  we  shall  describe  in  this  and 
the  following  chapters  are  : 


PROPERTIES    OF   MATTER.  29 

Extension  and  Figure. 

Impenetrability. 

Indestructibility. 

Inertia. 
Universal  Properties.  •(  _..  .  ..... 

Divisibility. 

Porosity. 

Compressibility  and  Expansibility. 
L  Elasticity. 

f  Hardness. 

Flexibility  and  Brittleness. 
Specific  Properties. . .  4  Tenacity. 

Malleability. 
I  Ductility. 

Besides  the  specific  properties  named,  there  are  others 
which  do  not  here  require  detailed  explanations,  such  as 
solidity  and  fluidity,  transparency  and  opacity,  color,  etc. 

The  universal  properties  are  sometimes  called  essential  properties  ;  that 
is,  properties  of  which  no  kind  or  form  of  matter  can  be  destitute.  The 
distinction  between  the  two  groups  named  will  be  explained  in  the  next 
section. 

11.  Extension  and  Figure. — Extension  is  that  property 
of  matter  by  virtue  of  which  a  body  occupies  a  limited 
portion  of  space  and  figure,  the  property  by  which  it  has 
some  definite  shape.  You  cannot  conceive  of  any  portion 
of  matter,  however  small  it  may  be,  that  does  not  occupy 
some  space,  and  that  has  not  shape  or  figure.  This  is,  in 
fact,  involved  in  the  very  idea  of  matter.  A  particle  may 
be  so  small  as  to  appear  only  as  a  point  to  the  naked  eye, 
but  viewed  through  the  microscope  its  shape  becomes 
obvious.  Even  an  atom  must  have  length,  breadth,  and 
thickness,  notwithstanding  our  inability  to  measure  it  or 
to  see  its  shape  with  the  most  powerful  microscope.  The 
air  is  sometimes  spoken  of  in  common  language  as  being 
shapeless.  This  is  partly  because  it  is  invisible,  and  partly 
because  no  portion  or  body  of  air  assumes  any  definite 

B 


30  NATUKAL   PHILOSOPHY. 

shape.  But  air  is  continually  forced  into  definite  shapes  by 
confinement  in  rooms,  boxes,  etc. ;  and  then  its  extension 
in  different  directions  can  be  measured  as  accurately  as  the 
extension  of  a  solid.  And,  besides,  the  particles  of  which 
air  is  composed  are  undoubtedly  solid,  and  we  cannot  con- 
ceive of  their  existence  without  attaching  to  them  the  idea 
of  figure  or  extension. 

Extension  is  an  essential  or  universal  property  of  matter,  since  no  por- 
tion of  matter,  hard  or  soft,  can  be  destitute  of  extension ;  on  the  other 
hand,  hardness  is  not  an  essential  property  of  matter,  for  some  kinds  of 
matter  possess  no  hardness. 

12.  Impenetrability. — In  common  language  one  substance 
is  said  to  penetrate  another  when  forced  into  it  by  pressure 
or  by  blows.  Thus  a  needle  penetrates  cloth,  a  nail  pene- 
trates wood,  etc.  But  this  .expression  is  not  strictly  cor- 
rect ;  the  needle  does  not  enter  the  substance  of  the  cloth, 
but  goes  between  the  fibres  of  it,  pushing  them  to  one  side. 
And  the  nail  goes  between  the  fibres  of  the  wood,  and  not 
into  them ;  it  does  not  occupy  the  same  room  as  the  fibres 
at  the  same  time. 

In  like  manner  no  atom  of  matter  can  penetrate  or  enter 
into  any  other  matter,  it  can  only  push  it  out  of  the  way, 
and  then  occupy  its  place.  This  property  of  matter  where- 
by no  two  portions  can  simultaneously  occupy  the  same 
place  is  called  impenetrability.  Impenetrability  is  usually 
accounted  one  of  the  universal  properties  of  matter,  but  it 
is  really  a  necessity  of  the  existence -of  matter  and  of  its 
extension,  as  explained  in  §  11. 

Illustrations. — The  property  of  impenetrability  may  be 
illustrated  in  many  ways.  If  you  hold  a  tumbler  with  its 
open  end  downward  and  press  it  into  water,  it  will  not  fill 
with  water,  for  the  air  in  the  tumbler  prevents  the  water 
from  rising;  it  cannot  occupy  the  same  space  with  the  air. 
It  fills  indeed  a  portion  of  the  tumbler,  because  the  air  is 


PROPERTIES    OF   MATTER. 


31 


compressible  to  a  certain  extent  (§  17).  If  you 
introduce  a  glass  funnel,  a,  Fig.  1,  into  a  jar  of 
water,  £,  the  water  will  not  rise  to  fill  it  so  long 
as  you  close  the  opening,  c,  with  your  finger. 
But  if  you  remove  your  finger,  the  water  will 
rise  to  the  level  of  the  water  outside  of  the  fun- 
nel, pushing  out  the  air  before  it.  The  follow- 
ing neat  experiment  illustrates  the  same  point. 
Float  a  lighted  candle,  a,  Fig.  2,  on  a  large  flat 

cork  (weighted  with  lead  so  as  to  support  the 
candle)  in  a  jar  of  water,  and  place  over  it  an 
open  jar  or  receiver,  &,  provided  with  a  stop- 
cock, c.  If  you  close  the  stopcock  and  press 
the  receiver  into  the  water,  the  candle  will 
sink  with  it  as  represented  in  the  figure,  the 
air  preventing  the 
water  from  entering 
the  jar.  As  soon  as 
you  open  the  stop- 
cock, however,  the 
water  will  rush  in,  and  the  can- 
dle will  appear  to  rise  out  of 
the  water. 

Other  Illustrations. — The  diving-bell, 
used  for  exploring  the  bottoms  of  rivers, 
lakes,  etc.,  affords  a  good  illustration 
of  this  property  of  matter.  It  consists 
of  a  metallic  vessel,  A,  Fig.  3,  shaped 
somewhat  like  a  bell,  and  made  suffi- 
ciently heavy  to  sink  in  water.  It  is 
lowered  into  deep  water  by  means  of  a 
chain  and  cable,  r  m  q.  The  water 
does  not  enter  the  bell  any  farther  than 
the  compressibility  of  the  air  allows. 
In  order  that  the  men  in  the  bell  may 


Fig.  2. 


32  NATURAL   PHILOSOPHY. 

remain  under  water  for  some  time,  fresh  air  is  supplied  by  the  tube  I,  being 
forced  in  by  a  pump,  and  the  vitiated  air  is  allowed  to  escape  through 
valves  provided  for  that  purpose.  The  seats,  s,  s,  are  for  the  accommodation 
of  the  men  who  descend  in  the  bell  to  work  at  the  bottom  of  the  sea  or  river. 
By  this  means  treasures  are  often  recovered  which  would  otherwise  be  lost. 
You  will  observe  some  resemblance  between  the  diving-bell  and  the  ar- 
rangement shown  in  Fig.  2,  the  receiver  representing  the  bell,  and  the 
lighted  taper  the  men  within. 

When  a  bullet  is  dropped  into  a  glass  of  water  it  pushes  the  particles  to 
one  side,  and  occupies  the  room  thus  gained.  If  several  bullets  are  thrown 
in,  there  is  an  evident  rise  of  the  water,  and  you  may  add  enough  to  make 
it  overflow.  The  same  thing  is  true  of  the  finest  needle  dropped  into  wa- 
ter ;  it  does  not  penetrate  the  water,  but,  like  the  bullet,  displaces  the  par- 
ticles. 

When  any  substance,  sugar  or  saltpetre  for  example,  is  dissolved  in  wa- 
ter, its  particles  do  not  penetrate  those  of  the  water,  but  they  enter  the 
spaces  between  the  particles.  In  more  concise  language,  solution  results 
from  inter-molecular  penetration.  In  like  manner  when  particles  of  odor- 
ous substances  are  diffused  in  the  air,  they  are  in  fact  between  its  particles. 

v  13.  Indestructibility. — We  have  alluded  to  the  indestruc- 
tibility of  matter  in  Chapter  I,  but  it  now  requires  ampli- 
fication. When  substances  waste  away,  or  are  burned  up 
rapidly,  the  matter  of  which  they  are  composed  does  not 
cease  to  exist — it  merely  changes  its  form  or  condition. 
Gold  may  be  melted,  and  even  converted  into  vapor,  but 
the  form  merely  changes,  and  its  substance  is  not  lost.  A 
candle*  grows  shorter  and  lighter  as  it  burns,  but  by  means 
of  suitable  apparatus  it  is  possible  to  collect  all  the  gases, 
smoke,  etc.,  which  invisibly  rise,  and  it  is  found  that  there 
is  no  loss  in  weight.  In  short,  we  cannot  create  matter, 
and  we  cannot  destroy  it ;  it  is  imperishable.  The  universe 
contains  the  same  amount  of  matter  as  when  first  called 
into  being  by  Omnipotence,  and  not  the  smallest  particle 
will  be  put  out  of  existence  to  the  end  of  time. 

We  occasionally  hear  of  new  elements,  new  kinds  of  matter,  dis- 
covered by  chemists ;  but  this  signifies  that  the  body  discovered  had  pre- 


PEOPEETIES    OF   MATTER.  33 

viously  escaped  observation,  either  on  account  of  its  great  rarity,  or  by 
reason  of  the  difficulty  of  distinguishing  it  from  its  associated  substances 
having  similar  properties.  New  elements  are  discovered,  not  created,  by 
chemists.  The  indestructibility  of  matter  is  forcibly  brought  to  the  mind 
of  the  chemist,  who  witnesses  in  his  dealings  with  matter  such  marvellous 
transformations,  disappearances,  and  reappearances.  The  changes  which 
the  various  kinds  of  matter  undergo  belong  to  the  province  of  Chemistry, 
and  will  be  fully  explained  in  Part  II. 

14.  Inertia. — Matter  has  no  power  to  put  itself  in  motion  ; 
when  it  is  moved,  it  is  acted  upon  by  some  force  outside  of 
the  matter,  or  communicated  to  it  in  some  way.  When  your 
arm  moves,  it  is  not  the  matter  in  your  arm  that  causes  its 
motion ;  it  is  the  result  of  a  force  within  you,  exerted  in 
obedience  to  the  will.  A  book  lying  on  a  table  has  no 
power  within  itself  of  moving  to  another  place ;  and  if 
you  reach  out  your  arm,  pick  it  up,  and  throw  it  across  the 
room,  you  communicate  force  to  it  from  without,  and  thus 
set  it  in  motion.  When  air  moves,  it  is  set  in  motion  by 
some  force  acting  upon  it,  as  when  you  blow  it  from  your 
lungs  or  move  it  with  a  fan.  When  the  wind  blows,  the 
air  is  set  in  motion  by  heat  and  the  attraction  of  the  earth, 
as  will  be  explained  in  another  part  of  this  book. 

Again,  matter  when  set  in  motion  has  no  power  to  stop 
itself.  This  inability  of  matter  to  move  itself  or  to  stop 
its  motion  is  called  inertia.  If  matter  could  stop  itself,  it 
would  not  be  called  inert.  Owing  to  this  inertness,  matter 
once  set  in  motion  would  keep  moving  forever  were  it  not 
stopped  by  some  force ;  matter  has  no  more  tendency  to 
stop  moving  when  once  put  in  motion  than  it  has  to  begin 
motion  when  it  is  at  rest.  All  motion  would  be  perpetual 
if  there  were  not  forces  opposing  it.  If  there  were  only 
one  body  in  the  universe,  and  that  were  set  in  motion,  it 
would  move  forever  through  empty  space  in  a  straight  line ; 
for  there  would  be  no  matter  anywhere  to  resist  its  mo- 
tion or  to  attract  it  away  from  its  onward  course. 


34  NATURAL  PHILOSOPHY. 

When  a  stone  falls  to  the  ground,  it  stops  simply  because  the  earth  ar- 
rests it.  If  the  earth  were  not  in  the  way,  the  stone  would  move  straight 
on  until  it  reached  the  centre  of  the  earth  (§  26).  A  stone  thrown  up 
in  the  air  would  keep  on,  and  soon  be  out  of  sight,  and  never  return  to  the 
earth,  if  it  were  not  made  to  come  down  by  forces  acting  upon  it.  One 
of  these  forces  is  the  resistance  of  the  air,  which,  from  the  moment  the 
stone  starts,  is  destroying  its  motion.  Another  force  as  constantly  operat- 
ing to  retard  the  stone  is  the  attraction,  or  drawing  force,  exerted  by  the 
earth  upon  it.  This  powerful  though  unseen' force  will  be  treated  of  fully 
in  Chapter  IV. 

Advantage  is  taken  of  inertia  in  matters  of  e very-day 
life.  The  massive  fly-wheel  of  a  stationary  engine,  once 
started  on  its  course,  continues  to  revolve  by  virtue  of  its 
inertia,  and  to  give  a  steady  regular  motion  to  the  machinery 
with  which  it  is  connected.  The  fly-wheel  is  made  very 
heavy,  because  the  heavier  it  is  the  greater  resistance  it 
offers  to  the  friction  which  continually  tends  to  stop  its 
motion.  When  the  locomotive  of  an  incoming  train  is  dis- 
connected and  shoots  swiftly  ahead,  the  train,  by  virtue  of 
its  inertia,  or  inability  to  stop  itself,  follows  after  until  its 
motion  is  spent  or  an  application  of  the  brakes  brings  it  to 
rest.  It  is  owing  to  the  inertia  of  matter  that  leaping  from 
a  rapidly  moving  train  is  dangerous;  a  person  on  the  cars 
partakes  of  their  motion,  and,  on  jumping  off,  his  feet  come 
suddenly  to  rest  while  his  body  continues  to  move  forward, 
and  he  is  thrown  headlong  to  the  ground. 
The  inertia  of  matter  may  be  illus- 
trated by  a  pleasing  experiment.  Bal- 
ance a  card  on  the  neck  of  a  bottle,  and 
place  a  small  coin  on  the  card  directly 
over  the  opening ;  by  giving  the  card 
a  quick,  sharp  blow  with  the  finger,  in 
a  horizontal  direction,  the  card  will  fly 
away  and  the  coin  will  fall  into  the  bot- 
Fig*4*  tie.  The  coin  does  not  move  with  the 


PROPERTIES    OP   MATTER.  35 

card,  because  sufficient  time  does  not  elapse  for  the  coin 
to  partake  of  its  motion. 

The  law  of  inertia  was  first  recognized  by  Galileo,  an  eminent  Italian 
philosopher,  about  the  close  of  the  sixteenth  century.  A  correct  compre- 
hension of  this  important  law  was  necessary  before  true  explanations  could 
be  given  of  the  laws  governing  the  falling  of  bodies,  of  the  vibrations  of 
the  pendulum,  and  of  the  motions  of  the  planets  in  their  orbits. 

15.  Divisibility  of  Matter. — Any  portion  of  matter,  wheth- 
er solid  or  gaseous,  may  be  divided  into  parts.  Even  if  it 
be  so  small  that  it  can  be  seen  only  with  a  powerful  micro- 
scope, it  could  be  still  further  subdivided,  provided  a  suffi- 
ciently delicate  instrument  were  available.  A  particle  of 
matter  the  five-thousandth  of  a  millimetre  in  diameter  is 
no  longer  visible  under  a  powerful  microscope,  and  yet 
nothing  but  man's  natural  incapabilities  prevents  the  divi- 
sion being  carried  yet  finer.  Whether  or  not  there  be  a 
limit  to  the  divisibility  of  matter  is  a  question  which  has 
been  discussed  by  philosophers  in  all  ages.  The  theory 
prevailing  at  present  is,  that  matter  is  not  infinitely  divis- 
ible, but  is  made  up  of  definite  ultimate  parts  called  atoms, 
as  explained  in  Chapter  I. 

Of  the  actual  divisibility  of  matter  we  have  numerous 
examples,  in  which  the  division  is  carried  far  beyond  that 
which  can  be  effected  by  any  cutting  instrument. 

A  gold-beater  can  hammer  a  grain  (65  milligrammes)  of 
gold  into  a  leaf  covering  a  space  of  fifty  square  inches  (322.5 
square  centimetres).  So  thin  is  it  that  it  would  take  300,000 
of  such  leaves,  laid  upon  each  other,  to  make  the  thickness 
of  an  inch.  And  yet  so  even  and  perfect  is  this  thin  layer 
of  gold,  that  when  laid  upon  any  surface  in  gilding  it  has 
the  appearance  of  solid  gold.  One  fifty-millionth  part  of 
this  grain  of  gold  thus  hammered  out  can  be  seen  by  the 
aid  of  a  microscope  which  magnifies  the  diameter  of  an 
object  ten  times. 


36  NATURAL   PHILOSOPHY. 

Recently  the  divisibility  of  gold  has  been  carried  much 
farther.  Mr.  Outerbridge,  of  Philadelphia,  has  obtained 
(by  electric  deposition)  films  of  gold  so  thin  that  one  grain 
of  the  metal  would  cover  nearly  four  square  feet  (0.37 
square  metre).  This  is  ten  thousand  times  thinner  than 
ordinary  writing-paper,  and  2,798,000  such  films  would 
measure  only  one  inch.  Gold-leaf  of  this  tenuity  is  trans- 
parent and  transmits  a  green  light. 

Further  Illustrations. — A  soap-bubble  is  a  beautiful  example  of  tbe 
minute  division  of  matter.  That  thin  wall  which  encloses  the  air  is 
composed  of  particles  of  the  soap  and  of  the  water  mingled  together.  It 
is  supposed  to  be  less  than  one  millionth  of  an  inch  in  thickness. 

The  thread  of  the  silk-worm  is  so  minute  that  the  finest  sewing-silk  is 
formed  of  many  of  these  threads  twisted  together.  But  the  spider  spins 
much  more  finely  than  this.  The  thread  by  which  he  lets  himself  down 
from  any  height  is  made  up  of  about  6000  threads  or  filaments,  each 
coming  from  a  separate  hole  in  his  spinning-machine.  A  quarter  of 
an  ounce  (7.77  grammes)  of  the  thread  of  a  spider's  web  would  extend 
400  miles  (643  kilometres). 

Platinum,  which  is  usually  regarded  as  the  heaviest  known  metal,  can  be 
drawn  out  into  wire  still  finer  than  the  web  of  the  spider ;  3000  feet  (914.4 
metres)  of  this  wire  weigh  scarcely  one  grain  (65  milligrammes),  and  a 
bundle  of  140  of  these  wires  would  equal  in  thickness  only  a  single  silk- 
worm thread.  See  §  22. 

Perhaps  the  most  minute  division  of  matter  is  exemplified  in  odors.  A 
grain  of  musk  will  scent  a  room  for  years,  and  yet  suffer  no  perceptible  loss 
of  weight.  But  during  all  this  time  the  air  is  filled  with  fine  particles 
coming  from  the  musk. 

The  microscope  reveals  to  us  many  wonderful  examples  of  the  minute- 
ness of  the  particles  of  matter,  both  in  the  vegetable. and  the  animal  world. 

If  you  press  a  common  puff-ball  a  dust  flies  off  like  smoke.  Examined 
with  a  microscope,  each  particle  of  this  dust,  which  is  the  seed  of  the  plant, 
is  a  perfectly  round  orange-colored  ball.  This  ball  is  of  course  made  up 
of  very  many  particles,  arranged  in  this  regular  form.  Beautiful  examples 
of  various  arrangements  of  the  minute  particles  of  matter  are  furnished 
by  the  pollen  of  different  plants,  as  seen  with  the  microscope. 

Each  particle  of  the  dust  which  adheres  to  yonr  fingers  as  you  catch  a 
moth  is  a  scale  with  fine  lines  upon  it  regularly  arranged.  And  if  you 


PROPERTIES    OP   MATTER.  37 

look  through  the  microscope  at  the  wing  of  the  moth,  you  will  see,  where 
the  scales  are  rubbed  off,  the  attachments  by  which  they  were  held 
standing  up  from  the  surface  of  the  wing,  like  nail-heads  on  a  roof  from 
which  the  shingles  have  been  torn. 

The  organization  of  exceedingly  small  animals,  as  re- 
vealed by  the  microscope,  furnishes  us  with  wonderful 
examples  of  the  minute  division  of  matter.  A  little  of  the 
dust  of  guano,  examined  through  a  powerful  microscope, 
is  seen  to  contain  multitudes  of  shells  of  various  shapes. 
These  shells  are  the  remains  of  animalcules  that  lived  in 
the  water,  their  destiny  seeming  to  be  in  part  to  furnish 
food  to  other  animals  larger  than  themselves.  In  the  chalk 
formations  of  the  earth  are  seen  multitudes  of  such  shells. 
They  have  been  discovered  even  in  the  glazing  of  a  visiting- 
card  ;  for  they  are  so  small  that  the  fine  grinding-up  of  the 
chalk  does  not  wholly  destroy  them.  There  are  animals, 
both  in  the  air  and  in  the  water,  so  small  that  it  would 
take  millions  of  them  to  equal  in  bulk  a  grain  of  sand,  and 
a  thousand  of  them  could  swim  side  by  side  through  the 
eye  of  a  common-sized  needle.  ISTow  all  these  animals  are 
furnished  with  organs,  constructed  of  particles  of  matter, 
which  are  arranged  in  them  with  as  much  order  and  sym- 
metry as  in  the  organs  of  our  bodies.  How  minute,  then, 
must  these  particles  be ! 

How  do  such  facts  extend  our  views  of  the  power  of 
the  Deity !  The  same  power  that  moulded  the  earth,  sun, 
moon,  and  the  whole  "  host  of  heaven,"  gave  form  and 
life  and  motion  to  the  millions  which  sport  in  every  sun- 
beam; the  same  eye  that  watches  the  immense  heavenly 
bodies  as  they  move  on  in  their  course  looks  upon  one 
and  all  of  these  legions  of  animals  in  earth,  air,  and  water, 
though  unseen  by  human  eyes,  and  provides  that  every 
particle  shall  take  its  right  position,  so  that  this  part  of 
creation  may  with  all  the  rest  be  pronounced  very  good ; 

B  2 


38  NATURAL  PHILOSOPHY. 

and  the  same  bountiful  hand  that  dispenses  the  means  of 
life  and  enjoyment  to  the  millions  of  the  human  race  for- 
gets not  to  minister  to  the  brief  life  and  enjoyment  of  each 
one  of  these  myriads  of  animalcules,  though  they  seem  to 
be  almost  nothingness  itself. 


QUESTIONS. 

10.  What  is  said  of  variety  in  the  properties  of  matter  ?  Give  the  clas- 
sification.— 11.  What  is  meant  by  extension?  What  by  figure?  Has 
the  air  any  extension  or  figure  ? — 12.  Illustrate  the  meaning  of  impene- 
trability. Describe  the  experiment  with  the  funnel  and  a  jar  of  water. 
Also  the  experiment  with  the  candle.  State  the  arrangement  of  the  div- 
ing-bell. Give  the  comparison  between  bullets  and  needles  in  relation  to 
penetration.  What  is  said  of  solution? — 13.  Explain  what  is  meant  by 
the  indestructibility  of  matter.  Can  we  create  matter?  What,  then,  is 
meant  by  new  elements  ? — 14.  What  is  inertia  ?  Give  illustrations  of  it. 
Illustrate  the  fact  that  matter  has  no  power  to  stop  its  own  motion.  What 
stops  a  body  set  in  motion  ?  Illustrate  by  reference  to  a  stone.  What 
advantage  is  taken  of  inertia.  Describe  an  experiment  illustrating  the 
inertia  of  matter.  Who  first  recognized  this  law? — 15.  What  is  said  of 
the  divisibility  of  matter  ?  Give  an  example  of  the  divisibility  of  matter 
by  reference  to  gold-leaf.  What  is  said  of  the  soap-bubble?  What  of 
the  thread  of  the  silk-worm,  and  of  the  web  of  the  spider?  What  of 
platinum  wires?  What  of  odors?  What  is  said  of  the  dust  of  the 
puff-ball  ?  What  of  pollen  ?  What  of  the  dust  rubbed  from  a  moth's 
wing  ?  What  of  guano  ?  What  of  the  glazing  of  visiting-cards  ?  What 
of  the  minuteness  of  some  animals  ?  What  is  said  of  the  Deity  in  re- 
lation to  minute  animals  ? 


PROPERTIES   OF   MATTER.  39 


CHAPTER  III. 

PROPERTIES   OF  MATTER    (CONTINUED). 

16.  Porosity. — The  particles  composing  bodies  of  every 
description  are  surrounded  by  empty  spaces;  those  sub- 
stances which  are  called  porous  have  quite  large  spaces  in 
them.  But  even  in  those  which  are  not  commonly  con- 
sidered porous  the  particles  are  by  no  means  close  to- 
gether. A  celebrated  experiment,  tried  at  Florence  in 
1661,  showed  that  the  particles  of  even  so  dense  a  sub- 
stance as  gold  are  separated  by  spaces  sufficiently  large  to 
let  water  pass  through  them.  A  hollow  golden  globe  con- 
taining water  was  subjected  to  great  pressure,  and  its  sur- 
face was  bedewed  with  the  water  that  came  out  through 
the  pores  of  the  gold. 

There  are  two  kinds  of  pores — sensible  pores,  which  can 
be  distinguished  by  the  naked  eye  or  by  the  aid  of  the 
microscope,  and  physical  pores,  or  interstices  among  the 
molecules  alluded  to  in  Chapter  I.  Sensible  pores  are  visi- 
ble in  wood,  sponge,  pumice-stone,  etc. ;  physical  pores  are 
invisible,  but  their  existence  is  shown  by  the  fact  that 
substances  can  be  compressed  into  a  smaller  bulk  than 
they  usually  occupy.  Solids  can  be  thus  compressed;  some 
more  than  others.  But  the  most  compressible  substances 
are  the  gases  and  vapors.  The  amount  of  space  between 
their  particles  must  be  very  large  to  allow  of  so  great  com- 
pression. 

\Ve  can  form  some  idea  of  the  great  amount  of  space  in  a  gaseous  or 
aeriform  substance  by  observing  the  difference  between  water  in  its  liquid 


40  NATURAL  PHILOSOPHY. 

and  in  its  aeriform  state.  A  cubic  cen- 
timetre of  water,  converted  into  steam, 
occupies  1696  times  more  room  than  be- 
fore. The  difference  in  proportion  is  ex- 
hibited in  Fig.  5,  the  inner  circle  repre- 
senting a  volume  of  water,  and  the  outer 
that  of  the  steam  into  which  it  is  con- 
verted. The  water  is  not  at  all  altered  in 
its  nature  by  being  changed  into  steam. 
The  particles  are  simply  forced  farther 
apart  by  the  heat,  and  as  soon  as  the 
Fig.  5.  heat  is  withdrawn  they  come  together 

again  to  form  water,  or,  in  other  words,  the  steam  is  condensed  into  water. 
It  is  plain,  therefore,  that  the  space  between  the  particles  is  1696  times  as 
great  in  steam  as  it  is  in  the  water  from  which  the  steam  is  made. 

When  any  substance,  as  sugar  or  salt,  is  dissolved  in 
water,  its  particles  are  diffused  through  the  intermolecu- 
lar  spaces.  In  like  manner,  when  water  evaporates,  the 
particles  of  water  are  diffused  through  the  spaces  between 
the  particles  of  the  air.  Animal  and  vegetable  bodies  are 
the  most  porous,  being  constituted  of  an  immense  number 
of  interlacing  channels  through  which  during  life  nourish- 
ing fluids  circulate.  This  is  evident  on  examination  of 
bone  and  of  wood  which  abound  in  cells  and  partitions. 

Density  and  Rarity. — The  density  of  a  substance  de- 
pends upon  the  quantity  of  matter  it  contains  in  a  given 
space.  The  more  dense,  therefore,  a  substance  is,  the 
greater  its  weight.  A  piece  of  lead  is  forty  times  heavier 
than  a  piece  of  cork  of  the  same  size.  Mercury  is  near- 
ly fourteen  times  heavier  than  an  equal  bulk  of  water. 
You  see,  then,  that  density  must  depend  on  the  nearness 
of  the  molecules  to  each  other.  In  so  dense  a  substance 
as  gold  the  molecules  are  all  very  close  together ;  in 
wood  there  are  spaces,  some  of  which  are  so  large  that 
you  can  see  them ;  and  in  air,  steam,  and  the  gases  there 


PROPERTIES    OF   MATTER.  41 

is  a  great  deal  of  space  among  the  particles,  so  that  we 
speak  of  their  rarity  instead  of  their  density. 

17.  Compressibility  and  Expansibility. — Owing  to  poros- 
ity, matter  may  be  compressed  and  expanded.  Pressure 
applied  to  porous  substances  brings  their  particles  nearer 
together,  making  them  fill  up  in  part  their  pores.  You 
have  a  very  familiar  example  of  this  in  sponge.  The 
greater  the  porosity  of  wood,  the  greater  its  compressi- 
bility. But  even  such  dense  substances  as  the  metals  can 
be  compressed  in  some  degree ;  that  is,  the  interstices  be- 
tween their  particles  can  be  made  smaller.  Medals  and 
coins  have  their  figures  and  letters  stamped  upon  them 
by  pressure,  just  as  impressions  are  made  upon  melted 
sealing-wax.  The  heavy  and  quick  pressure  required  to 
do  this  actually  compresses  the  whole  piece  of  the  hard 
metal,  putting  all  the  particles  nearer  together,  so  that 
it  occupies  less  space  than  it  did  before  it  was  stamped. 

It  might  be  supposed  from  the  freencss  with  which  the 
particles  of  liquids  move  among  each  other,  and  from  the 
spaces  which  exist  among  them,  that  these  substances 
could  be  easily  compressed.  But  it  is  not  so.  The  heavi- 
est pressure  is  required  to  compress  them  even  in  a  slight 
degree.  Water  can  be  compressed  so  very  little  that  prac- 
tically it  is  regarded  as  incompressible. 

Although  the  interstices  between  the  particles  of -liquids  cannot  be 
varied  by  mechanical  pressure,  they  can  be  by  variations  of  temperature. 
Liquids  are  dilated  or  expanded  by  heat ;  that  is,  their  particles  are  put 
farther  apart.  They  are  contracted  or  compressed  by  cold ;  that  is,  their 
particles  are  brought  nearer  together  by  the  abstraction  of  heat.  The 
most  familiar  example  is  the  thermometer.  The  mercury  rises  in  the 
tube  when  the  heat  increases  the  interstices  between  its  particles,  and 
it  falls  when  the  loss  of  heat  allows  the  particles  to  come  near  together. 

Aeriform  bodies  are  more  compressible  than  any  other 
substances,  showing  that  in  their  ordinary  condition  there 


42  NATURAL   PHILOSOPHY. 

is  a  great  deal  of  space  among  their  particles.  While  they 
are  thus  unlike  liquids  in  compressibility,  they  are  affected 
by  heat  in  the  same  way. 

18.  Elasticity. — Closely  allied  to  the  compressibility  of 
matter  is  its  elasticity.  We  see  this  property  strikingly 
exemplified  in  India-rubber.  It  occasions  the  rebounding 
of  a  ball  of  this  substance  when  thrown  against  any 
immovable  body — the  floor,  for  example.  When  the  ball 

meets  the  resistance  of  the 
floor  it  is  flattened,  as  rep- 
resented in  Fig.  6.  Then, 
as  it  assumes  the  round 
shape,  shown  in  Fig.  V,  it 
Flg-6-  Fig.  7.  pushes  downward  upon  the 

floor.  .  It  is  this  sudden  pushing  downward  that  makes  it 
rebound.  It  is  as  if  there  were  a  compressed  spring  be- 
tween the  ball  and  floor.  It  may  be  likened  also  to  jump- 
ing. When  a  person  jumps  he  bends  his  limbs  at  the  thigh 
and  knee-joints,  and  then,  in  straightening  himself  up,  gives 
a  sudden  push,  like  that  given  by  the  ball  as  it  assumes  its 
round  shape,  and  so  is  thrown  forward  or  upward,  accord- 
ing to  the  direction  of  the  pushing  force.  The  same  flat- 
tening occurs  in  an  ivory  ball,  though  to  a  far  less  de- 
gree. You  can  prove  this  by  experiment.  Let  a  marble 
slab  be  moistened,  and  drop  the  ball  upon  it.  Quite  a 
spot  will  be  made  dry  by  the  blow  of  the  ball,  showing 
that  it  touched  more  of  the  marble  than  it  does  when 
merely  placed  upon  it. 

When  a  stick  or  rod  is  bent,  as  soon  as  the  bending  force  is  withdrawn 
the  stick  becomes  straight  again  from  its  elasticity.  It  is  this  elastic  force 
of  the  bow,  straightening  it,  that  speeds  the  arrow.  Observe  in  this  case 
that  while  the  particles  on  the  concave  side  of  the  bent  bow  are  brought 
nearer  together,  or  compressed,  those  on  the  convex  side  are  moved  apart. 
This  moving  apart  of  the  particles  is  often  shown  in  India-rubber.  You 
will  see  how  very  far  apart  particles  in  near  neighborhood  may  be  carried 


PROPERTIES   OP   MATTER.  43 

if  you  stick  two  pins  close  together  in  a  strip  of  India-rubber,  and  observe 
their  movements  when  you  stretch  it. 

Some  substances  have  so  very  little  elasticity  that  practically  they  are 
considered  as  having  none.  Lead  is  one  of  these.  A  rod  of  lead  when 
bent  remains  so,  and  a  leaden  ball  does  not  rebound.  While  aeriform 
substances  are  the  most  compressible  of  all,  they  are  also  the  most  elastic. 
Compressed  air  returns  to  its  usual  condition  the  moment  it  is  relieved 
from  the  pressure,  and  with  a  force  proportioned  to  the  amount  of  the 
pressure.  So  it  is  with  steam  and  the  gases.  The  varied  results  of 
this  property  of  aeriform  substances  will  claim  our  attention  more  par- 
ticularly in  some  other  parts  of  this  book. 

Glass  is  nearly  perfectly  elastic — that  is,  it  will  retain  no 
permanent  bend ;  but  the  limits  of  its  elasticity  are  very 
small:  it  will  not  bend  far  without  breaking.  Hard  bodies 
in  general  have  a  much  smaller  elastic  limit  than  soft  ones. 
This  is  evident  on  comparing  the  elasticity  of  steel,  ivory, 
stone,  glass,  etc.,  with  that  of  silk,  catgut,  India-rubber, 
and  the  like. 

Elasticity  may  be  defined  as  that  property  of  matter  by 
which  its  particles,  when  brought  nearer  together  or  car- 
ried farther  apart  by  any  force,  return  to  their  usual  con- 
dition when  the  force  is  withdrawn.  Closely  connected 
with  elasticity  is  the  property  of  flexibility,  which  will  be 
explained  in  the  next  section. 

\  19.  Flexibility  and  Brittleness. — If  you  bend  a  flexible 
body — a  piece  of  wood,  for  ex- 
ample— it  is  obvious  that  the 
particles  on  the  upper  or  con- 
vex side  must  be  forced  a  lit-  Fis- 8- 
tie  farther  apart,  while  those  on  the  under  or  concave  side 
are  brought  a  little  nearer  together  (Fig.  8).  But  the 
wood  does  not  break,  because  the  particles  that  are  thus 
moved  a  little  apart  still  retain  their  hold  upon  each  oth- 
er. This  is  the  explanation  of  what  is  called  flexibility. 
On  the  other  hand,  the  particles  in  a  rod  of  glass  cannot 


44  NATURAL   PHILOSOPHY. 

be  put  farther  apart  in  this  way.  They  are  not  actually 
in  contact  any  more  than  are  the  particles  of  the  wood,  but 
they  are  in  a  fixed  .relative  position  ;  that  is,  a  position 
which  cannot  be  disturbed  without  &  permanent  separation 
of  particles.  If  you  attempt  to  bend  the  rod  there  is  no 
slight  separation  of  many  particles,  as  in  the  bent  wood, 
but  a  full  and  permanent  separation  in  some  one  part  of  the 
rod.  We  call  the  property  on  which  this  result  depends 
brittleness.  Brittle  substances  are  generally  hard.  Glass, 
while  the  most  brittle  of  all  substances,  is  hard  enough  to 
scratch  iron. 

Steel. — There  are  two  kinds  of  steel,  flexible  and  brittle.  The  steel  of 
most  cutting  instruments*  is  brittle.  The  steel  of  a  sword-blade  is  quite 
flexible,  and  that  of  a  watch-spring  is  so  much  so  that  we  can  wind  it  up 
in  a  coil.  This  difference  is  owing  to  a  difference  in  the  mode  of  cooling 
the  steel.  If  it  be  cooled  suddenly,  it  is  brittle ;  if  slowly,  it  is  flexible. 
The  process  by  which  it  is  cooled  slowly  is  called  annealing.  The  expla- 
nation of  all  this  is  quite  simple.  The  steel  being  expanded  by  heat — that 
is,  its  particles  being  put  farther  apart  than  they  usually  are — when  they 
are  suddenly  brought  together  again  they  have  not  time  to  arrange  their 
relative  position  properly.  Brittleness  is,  therefore,  the  result.  But,  on 
the  other  hand,  when  the  cooling  is  effected  gradually,  time  is  given  for 
the  arrangement. 

Steel  suddenly  hardened  is  too  brittle  for  common  use.  A  process  called 
tempering  is  therefore  resorted  to  for  diminishing  the  brittleness.  The 
steel  is  reheated  after  the  hardening,  and  is  then  allowed  to  cool  slowly. 
The  degree  in  which  the  brittleness  is  lessened  depends  on  the  degree  of 
heat  to  which  the  steel  is  subjected.  It  can  be  entirely  removed  by  a  red 
heat,  for  then  the  particles  have  a  full  opportunity  to  readjust  themselves; 
and  the  more  the  heat  comes  short  of  this  point  the  less  thorough  will  be 
the  adjustment,  because  the  less  perfectly  are  the  particles  released  from 
their  suddenly  taken  position.  In  lessening  the  brittleness  we  diminish 
hardness  also,  and  therefore  the  tempering  is  varied  in  different  cases  ac- 
cording to  the  degree  of  hardness  desired. 

Annealing  of  Glass. — Glass  for  economical  uses  is  always 
annealed.  If  this  were  not  done  our  glass  vessels  and  win- 
dow-panes would  be  exceedingly  brittle,  and  be  constantly 


PROPERTIES    OF   MATTER.  45 

breaking.  Articles  made  of  glass  are  annealed  by  being 
passed  very  slowly  through  a  long  oven  which  is  very  hot 
at  one  end,  the  heat  gradually  lessening  towards  the  other 
end. 

We  have  a  striking  example  of  brittleness  induced  by 
sudden  cooling  in  what  are  called  "Prince  Rupert's  Drops." 
These  are  made  by  dropping  melted  glass  into  cold  water, 
and  they  usually  have  a  shape  resem- 
bling that  of  Fig.  9.  If  you  break  off  ever 
so  small  a  bit  of  the  point  of  one  of  these 
drops,  the  whole  will  at  once  shiver  to 
pieces.  That  is,  the  sudden  arrangement 
of  the  particles  is  so  slight  and  unnatural 
that  the  disturbance  of  the  arrangement 
in  a  small  part  suffices  to  destroy  the 
arrangement  of  the  whole,  very  much  as 
a  row  of  bricks  is  thrown  down  by  the 
fall  of  the  first  in  the  row.  Faraday  says 
that  those  drops  were  not,  as  is  commonly 
supposed,  invented  by  Prince  Rupert,  but 
were  first  brought  to  England  by  him  in 
1G60.  They  excited  much  curiosity  at  that  time,  and  were 
«  considered  "a  kind  of  miracle  in  nature."  But  you  see 
that  this,  like  many  other  wonders,  is  capable  of  an  easy 
explanation. 

The  so-called  "tempered  glass,"  invented  by  a  Frenchman  named  La 
Bastie,  affords  another  example.  La  Bastie's  process  consists  in  heating 
the  glass  to  a  certain  temperature,  and  passing  it  through  oil  or  fatty  ma- 
terials ;  glass  articles  thus  treated  are  rendered  tough  enough  to  stand 
rough  usage,  such  as  dropping  on  a  wooden  floor  from  a  height  of  ten  feet, 
and  even  hammering  to  a  certain  extent.  And  yet  the  glass  is  in  a  pe- 
culiar condition,  and,  when  broken,  is  shivered  into  thousands  of  pieces, 
much  as  is  the  case  with  Prince  llupert's  Drops. 

20.  Hardness. — This  property  seems  to  depend  upon  some 


46  NATURAL   PHILOSOPHY. 

peculiar  arrangement  of  the  particles  of  matter.  We  should 
suppose  that  the  densest  substances  would  be  the  hardest. 
But  it  is  not  so.  Iron  is  the  hardest  of  the  metals,  but  its 
particles  are  not  so  close  together  as  those  of  gold,  which 
is  quite  a  soft  metal.  And  gold  is  about  four  times  as 
heavy  as  the  diamond,  which  is  so  hard  as  to  cut  glass 
easily.  Common  flint  is  hard  enough  to  scratch  glass,  but 
will  not  cut  it  so  well  as  the  diamond. 

Advantage  is  taken  of  the  different  degrees  of  hardness 
possessed  by  minerals  in  determining  their  species.  In  the 
following  table  a  number  of  minerals,  whose  degrees  of 
hardness  is  very  uniform,  are  arranged  so  as  to  form  a  con- 
venient scale,  by  reference  to  which  the  hardness  of  any 
substance  can  be  determined.  It  is  only  necessary  to  secure 
specimens  of  the  minerals  named,  and  to  ascertain  which 
of  these  ten  the  body  under  trial  will  scratch.  Since,  how- 
ever, it  is  not  always  possible  to  obtain  a  complete  set  of 
these  minerals,  we  have  added  remarks  showing  approxi- 
mately their  hardness : 

SCALE   OF   HARDNESS. 

1.  Talc  ;  easily  scratched  by  the  finger-nail. 

2.  Gypsum ;  not  easily  scratched  by  the  nail ;  does  not  scratch  a  copper 

coin.  -    < 

3.  Pure  limestone  (calcite) ;  is  scratched  by  a  copper  coin. 

4.  Fluor-spar ;  not  scratched  by  a  copper  coin. 

5.  Apatite ;  scratches  glass  with  difficulty,  but  is  easily  scratched  by  a 

knife. 
G.  Feldspar ;  scratches  glass  easily ;  is  scarcely  scratched  by  a  knife. 

7.  Quartz  ;  not  scratched  by  a  knife. 

8.  Topaz  ;  harder  than  quartz. 

9.  Corundum ;  harder  still. 

10.  Diamond ;  is  scratched  by  no  other  substance. 
The  property  of  hardness  depends  on  some  circumstances  not  perfectly 
understood,  for  a- substance  may  be  hard  or  soft  according  to  the  manner 
in  which  it  is  treated.    That  this  is  the  case  with  steel  has  been  mentioned 


PROPERTIES    OP    MATTER.  47 

21.  Tenacity. — The  power  possessed  by  substances  which 
causes  them  to  resist  being  pulled  asunder,  termed  tenac- 
ity, depends  on  the  degree  of  attraction  between  the  par- 
ticles. By  attraction  we  mean  a  disposition  in  particles 
to  come  together,  this  disposition  being  manifested  in  op- 
position to  any  force  tending  to  draw  them  apart.  Tenac- 
ity does  not  exist  at  all  in  gaseous  substances.  The  par- 
ticles of  air  and  of  steam,  for  example,  show  no  disposition 
to  cling  together — that  is,  have  no  tenacity.  This  property 
is  weak  in  liquids;  it  is  only  strong  enough  in  water  to 
enable  its  particles  to  hang  together  in  the  shape  of  a  drop. 
It  is  strong  in  solids,  enabling  their  particles  not  only  to 
hold  together  in  large  quantities,  but  also  to  hold  up  heavy 
weights  suspended  to  them.  It  is  strongest  of  all  in  steel. 

Various  metals  and  other  substances  have  been  tested 
to  ascertain  their  comparative  tenacity.  It  was  done  in 
this  way :  Rods  were  made  of  the  metals,  etc.,  all  of  the 
same  size,  having,  in  fact,  a  cross-section  of  one  square  inch. 
Weights  were  suspended  to  them,  and  additions  were  made 
to  the  weights  little  by  little  till  the  rods  broke.  The 
table  below  was  made  by  placing  against  each  substance 
the  greatest  weight  that  its  rod  would  sustain : 

TABLE    SHOWING   COMPARATIVE   TENACITY   OF  MATERIALS. 

Cast  steel 45  to  60  tons. 

Best  wrought  iron •.  25  to  30    " 

Cast  iron C  to  13    " 

Copper 9to26    ' 

Platinum 8 

Ash-wood 8 

Silven 5 

Beech-wood 5 

Gold..... 4J- 

Zinc 2  tons. 

Tin about    H  to  2    " 

Lead..  1  ton. 


48  NATUKAL  PHILOSOPHY. 

Some  animal  substances  have  great  tenacity,  as  the 
thread  of  the  silk -worm,  hair,  wool,  and  the  ligaments 
and  tendons  of  our  bodies  and  of  other  animals. 

"The  gradual  discovery,"  says  Dr.  Arnott,  " of  substances  possessed 
of  strong  tenacity,  and  which  man  could  yet  easily  mould  and  apply  to  his 
purposes,  has  been  of  great  importance  to  his  progress  in  the  arts  of  life. 
The  place  of  the  hempen  cordage  of  European  navies  is  still  held  in  China 
by  twisted  canes  and  strips  of  bamboo ;  and  even  the  hempen  cable  of 
Europe,  so  great  an  improvement  on  former  usage,  is  now  rapidly  giving 
way  to  the  more  complete  and  commodious  security  of  the  iron  chain— of 
which  the  material  to  our  remote  ancestors  existed  only  as  useless  stone  or 
earth.  And  what  a  magnificent  spectacle  it  is,  at  the  present  day,  to  be- 
hold chains  of  tenacious  iron  stretched  high  across  a  channel  of  the  ocean, 
as  at  the  Menai  Strait  between  Anglesea  and  England,  and  supporting  an 
admirable  bridge-road  of  safety,  along  which  crowded  processions  may  pour, 
regardless  of  the  deep  below  or  of  the  storm ;  and  beneath  which  ships, 
with  sails  full-spread,  pursue  their  course  unmolesting  and  unmolested." 

22.  Malleability  and  Ductility.— Those  metals  which  can 
be  hammered  into  thin  plates  are  called  malleable.  Gold 
furnishes  us  with  the  best  illustration  of  this  property. 
We  have  already  mentioned  (§  15),  that  a  single  grain 
of  gold  can  be  hammered  out  into  a  sheet  the  one 
300,000th  part  of  an  inch  in  thickness.  An  alloy  of  20 
parts  of  gold  and  22  of  silver  is  equally  malleable.  Silver, 
copper,  and  tin  are  quite  malleable;  but  the  thinnest  leaves 
of  tin  are  the  one  1600th  part  of  an  inch  in  thickness. 
Most  of  the  other  metals  are  very  little  so,  and  some  of 
them  are  not  at  all,  breaking  at  the  first  blow.  A  substance 
is  said  to  be  ductile  when  it  can  be  drawn  out  into  wire. 
The  principal  metals  that  have  this  quality  are  platinum, 
silver,  iron,  copper,  and  gold,  and  in  the  order  named. 
The  celebrated  English  chemist  Dr.Wollaston  obtained  a 
platinum  wire  only  the  one  30,000th  part  of  an  inch  in  di- 
ameter by  the  following  ingenious  process.  A  small  plat- 
inum wire  was  soldered  within  a  cylinder  of  silver,  and 


PEOPEETIES   OF  MATTEE.  49 

the  compound  wire  was  drawn  out  in  the  usual  way  as 
fine  as  possible.  The  silver  was  then  dissolved  off  by 
immersing  the  wire  in  nitric  acid,  and  the  platinum  core 
remained  about  half  the  thickness  of  the  thread  of  a 
spider's  web.  Melted  glass  is  very  ductile.  It  can  be 
drawn  out  into  a  very  fine  thread ;  and  when  this  thread  is 
cut  and  arranged  in  bunches,  it  resembles  beautiful  white 
hair.  In  hammering  metals  into  plates,  or  drawing  them 
into  wire,  there  is  a  considerable  change  of  relative  posi- 
tion in  the  particles,  similar  to  that  which  we  have  in 
fluids,  though  nothing  like  so  free.  In  this  change  of 
position,  those  particles  that  remain  in  close  neighbor- 
hood have  a  remarkable  tenacity  or  attraction,  preventing 
their  separation.  In  welding  two  pieces  of  iron,  which  is 
done  by  the  blacksmith  by  hammering  them  together 
when  red-hot,  there  must  be  enough  movement  among 
the  particles  to  permit  those  of  one  piece  to  mingle  some- 
what with  those  of  the  other. 

23.  Usefulness  of  Variety  in  Properties  of  Matter. — The  vari- 
ous properties  of  matter  brought  to  view  in  this  and  the  preceding  chap- 
ters are  providential  adaptations  to  the  necessities  of  man.  Each  substance 
has  those  properties  which  best  fit  it  for  his  use.  Iron,  for  example,  de- 
signed by  the  Creator  to  be  both  the  strongest  and  most  extensively  useful 
servant  of  man  among  the  metals,  is  therefore  provided  in  great  abundance, 
and  has  those  strong,  decided,  and  various  qualities  which  fit  it  for  the 
services  it  is  to  perform.  Gold  and  silver,  on  the  other  hand,  designed  for 
services  less  extensive,  and  in  a  great  measure  ornamental,  are  provided  in 
very  much  less  quantity,  and  have  properties  admirably  adapting  them  to 
the  services  for  which  they  are  so  manifestly  intended.  The  same  can 
be  substantially  said  of  all  other  substances,  and  especially  of  those  very 
abundant  ones  air  and  water.  And  it  may  be  remarked  also  that  the 
ingenuity  of  man  is  continually  discovering  new  modes  of  bringing  the 
various  properties  of  matter  into  his  service.  We  will  give  but  a  single  il- 
lustration—the tempering  of  steel.  "This  discovery,"  says  Dr.  Arnott, 
"is  perhaps  second  in  importance  to  few  discoveries  which  man  has  made; 
for  it  has  given  him  all  the  edge-tools  and  cutting  instruments  by  which 


50  ,   NATUllAL   PHILOSOPHY. 

he  now  moulds  every  other  substance  to  his  wishes,  and  to  which  he  owes 
all  his  modern  mechanical  improvements.  A  savage  would  work  for 
twelve  months  with  fire  and  sharpened  flints  to  fell  a  great  tree  or  carve 
a  rough  canoe,  where  a  modern  carpenter,  with  his  tools  of  hard  steel, 
could  accomplish  the  same  object  better  in  a  few  days." 


QUESTIONS. 

1C.  What  is  meant  by  the  porosity  of  matter?  Show  that  gold  is 
porous.  Name  the  kinds  of  pores,  and  explain  by  illustrations.  What 
proof  is  there  that  all  substances  have  spaces  in  them  ?  What  is  said  of 
the  amount  of  space  in  gases  and  vapors?  Give  the  statement  in  regard 
to  steam.  What  is  said  of  solutions  of  solids  in  fluids  ?  What  of  evapo- 
ration ?  What  of  animal  and  vegetable  bodies  ?  Upon  what  do  density 
and  rarity  depend  ? — 17.  What  is  said  of  compressibility  ?  Illustrate. 
What  of  the  incompressibility  of  liquids  ?  How  is  the  position  of  the 
particles  of  liquids  affected  by  a  change  of  temperature  ?  Which  are  the 
most  compressible  substances? — 18.  Explain  elasticity  by  reference  to  a 
rubber-ball.  Illustrate  by  reference  to  a  bent  stick.  WThat  is  said  of 
the  degrees  of  elasticity  in  different  substances?  Define  elasticity. — 10.  Il- 
lustrate what  is  meant  by  flexibility.  What  of  brittleness?  Give  ex- 
amples of  flexible  and  brittle  steel.  Explain  the  actual  difference  be- 
tween them.  Explain  the  tempering  of  steel.  What  is  said  of  the  an- 
nealing of  glass  ?  What  of  Prince  Rupert's  Drops  ?  What  of  "tempered 
glass  ?" — 20.  Upon  what  does  the  hardness  of  bodies  depend  ?  What  use 
is  made  of  the  different  degrees  of  hardness  in  minerals?  Name  the 
typical  minerals. — 21.  Define  tenacity.  What  is  said  of  the  comparative 
tenacity  of  substances  ?  Which  metal  is  the  strongest  ?  Which  the 
weakest?  What  is  said  of  the  value  of  tenacious  bodies? — 22.  What 
is  the  difference  between  malleability  and  ductility  ?  Give  examples. — 
23.  What  is  said  of  the  usefulness  of  the  variety  of  properties  in  matter  ? 
What  of  the  importance  of  steel  ? 


ATTRACTIONS   OF   MATTER.  51 


CHAPTER  IV. 

ATTRACTIONS    OF   MATTER. 

24.  Matter  attracts  Matter. — We  have  already  stated 
that  matter  is  acted  upon  by  forces,  and  we  will  now  ex- 
plain this  more  fully.  The  minute  particles  of  matter  of 
which  bodies  are  composed  do  not  touch  each  other,  but 
even  in  the  densest  substances  are  surrounded  by  void 
spaces,  and  these  particles  are  held  in  their  place  by  at- 
traction between  them,  each  particle  of  matter  attracting 
every  other  particle.  This  property  invariably  accom- 
panies matter  of  every  form  and  under  all  circumstances. 
And  since  tangible  masses  are  made  up  of  small  particles, 
what  is  true  of  the  latter  is  equally  true  of  the  former. 
Every  body  in  the  universe  attracts  with  greater  or  less 
force  every  other  body,  however  near  or  distant  they  may 
•be  from  each  other.  Sun,  earth,  moon,  and  stars  attract 
each  other;  and  this  power  binds  them  together  as  they  roll 
through  space.  This  force  is  generally  called  the  attrac- 
tion of  gravitation,  a  name  given  to  it  because  we  have  such 
common  examples  of  its  influence  in  the  fall  of  bodies  jto 
the  earth ;  they  are  said  to  gravitate  towards  the  earth. 

Whether  the  mysterious  force  which  binds  the  minute 
particles  of  matter  together  to  constitute  masses  is  the 
same  as  that  which  controls  the  motions  of  celestial 
bodies  is  as  yet  unproved.  That  an  attraction  actually 
exists  between  small  masses  when  they  are  brought  ex- 
ceedingly close  to  each  other  is  easily  shown.  Thus  if 
two  cork  balls  coated  with  varnish  be  placed  on  the  sur- 
face of  water  near  to  each  other,  their  attraction  will  soon 


52  NATUJJAL   PHILOSOPHY. 

bring  them  together.  Thin  globes  of  glass  will  exhibit 
the  same  attraction.  So,  also,  floating  pieces  of  wood  are 
apt  to  be  found  together ;  and  when  a  ship  is  wrecked,  the 
parts  of  the  wreck  collect  in  tangled  masses  here  and  there 
on  the  surface  of  the  sea  as  soon  as  it  becomes  calm.  The 
gravitation  between  particles  and  masses  of  matter  may 
possibly  be  identical,  and  our  appreciation  of  it  depends 
upon  their  relative  size  and  distance.  For  convenience 
of  distinction,  different  names  have  been  given  to  attrac- 
tion according  to  the  distances  at  whiclrit  acts. 

Gravitation  is  the  attraction  existing  between  matter 
at  great  or  appreciable  distances,  as  between  the  heavenly 
bodies,  or  between  the  earth  and  a  stone  thrown  into  the  air. 

Cohesion  is  the  attraction  between  molecules  of  the  same 
kind  of  matter  binding  them  together  to  form  masses. 

Adhesion  is  the  attraction  between  molecules  of  dis- 
similar matter,  as  exhibited  in  cements.  A  peculiar  kind 
of  adhesion  is  known  as  capillary  attraction. 

Chemical  attraction  is  the  force  which  binds  together 
the  atoms  of  a  molecule  (§  8).  Its  study  is  the  province 
of  chemistry  and  will  be  fully  treated  in  Part  II. 

Explanations  and  illustrations  of  the  phenomena  result- 
ing from  these  attractions  will  occupy  the  remainder  of 
this  chapter  and  the  succeeding  one. 

25.  Gravitation. — A  stone  falls  to  the  ground  for  precisely 
the  same  reason  that  the  two  cork  balls  approach  each 
other  when  floated  on  water  (§  24).  It  falls  owing  to  the 
attraction  which  the  earth  and  the  stone  have  for  each 
other;  in  other  words,  the  attraction  is  mutual.  If  you 
hold  a  stone  in  your  hand  and  thus  prevent  its  falling,  you 
simply  resist  a  power  which  is  pulling  it  down.  If  it  were 
possible  to  suspend  the  mutual  attraction  of  the  earth  and 
the  stone,  you  could  release  your  hold  of  the  stone,  and  it 
would  remain  suspended  in  the  air  until  the  attraction  had 


ATTK ACTIONS    OP   MATTER.  53 

been  restored.  The  attraction  of  the  earth  and  the  stone  is 
really  mutual;  but  the  earth  is  so  immense  in  comparison 
with  the  stone  that  its  motion  towards  the  stone  is  exceed- 
ingly small,  and  may  practically  be  considered  as  naught. 

This  may  be  clearly  illustrated  by  a  comparison  of  the  force  of  attraction 
with  the  force  of  muscular  action.  Suppose  a  man  in  a  boat  pulls  on  a 
rope  which  is  made  fast  to  a  ship  lying  loose  at  the  wharf,  and  in  this  way 
draws  his  boat  towards  it.  He  does  not  consider  that  he  moves  the  ship  at 
all ;  but  in  reality  he  does,  for  if,  instead  of  one,  a  hundred  or  more  men  in 
boats  pull  upon  the  ship,  they  will  make  the  motion  apparent.  In  the  case 
of  the  single  boat,  the  motion  of  the  ship  is  as  real  as  when  a  hundred  boats 
are  pulling  it,  but  it  is  only  tlie  one-hundredth  part  as  great.  Now  let 
the  ship  represent  the  earth,  and  the  little  boat  some  object,  as  a  stone,  at- 
tracted by  it.  The  earth  and  the  stone  move  towards  each  other,  just  as 
the  ship  and  the  boat  do.  And  if,  as  we  multiplied  the  number  of  boats, 
we  should  multiply  the  bulk  of  the  stone  till  it  is  of  an  immense  size,  it 
would  by  its  attraction  have  a  perceptible  influence  upon  the  earth. 

Observe  in  regard  to  the  illustration  that  it  makes  no  difference  whether 
the  man  pull  in  the  boat  or  in  the  ship.  In  either  case  he  exerts  an 
equal  force  on  the  ship  and  the  boat,  making  them  to  approach  each  other. 
So  it  is  with  the  attraction  between  the  earth  and  the  stone.  It  is  a  force 
exerted  equally  upon  both.  Its  effect  on  the  earth  is  not  manifest,  because 
it  is  so  much  larger  than  the  stone;  just  as  the  effect  of  the  man's  exertion 
is  not  manifest  upon  the  ship,  because  it  is  so  much  larger  than  the  boat. 

Proportion  of  the  Mutual  Motions  of  Attraction. — Let  us  pursue  the 
illustration  a  little  farther.  If  a  man  stand  in  a  boat,  and  pull  a  rope  made 
fast  to  another  boat  of  the  same  size  and  weight,  both  boats,  in  coming  to- 
gether, will  move  over  the  same  space.  Just  so  rt  is  with  the  attraction 
between  two  bodies  having  the  same  quantities  of  matter  or  equal  masses 
— they  attract  each  other  equally,  and  therefore  meet  each  other  half-way. 
Suppose,  however,  that  one  boat  is  ten  times  as  great  and  as  heavy  as  the 
other.  The  small  boat  would  move  ten  times  as  much  as  the  large  one 
when  the  man  brings  them  together  by  pulling  the  rope.  In  like  manner, 
if  a  body  one  tenth  as  large  as  the  earth  should  approach  it,  they  would 
attract  each  other,  but  in  coming  together  the  body  would  move  ten  times 
as  far  as  the  earth.  In  the  case  of  falling  bodies,  even  though  they  may 
be  of  great  size,  the  earth  moves  so  slightly  to  meet  them  that  its  motion 
is  wholly  imperceptible.  It  has  been  calculated  that  if  a  ball  of  earth  the 

c 


NATURAL   PHILOSOPHY. 


tenth  of  a  mile  in  diameter  were  placed  at  the  distance  of  a  tenth  part 
of  a  mile  from  the  earth,  and  let  fall,  the  motion  of  the  earth  would  be  only 
the  one  eighty-thousand-millionth  (^o^^o'Vo^  ow)  Part  °f  an  inch. 

26.  Attraction  Towards  the  Earth's  Centre. — All  bodies 
are  attracted  towards  the  centre  of  the  earth.  This  is 
because  the  earth  is  spherical.  Let 
the  circle,  Fig.  10,  represent  the 
earth,  and  a  a  body  attracted  by  it. 
The  lines  drawn  from  the  body  to 
the  earth  represent  the  attractive 
force  exerted  by  the  earth  upon  the 
body.  It  is  obvious  from  these  that 
there  is  as  much  attraction  on  the 
one  side  of  the  line  drawn  from  the 
body  to  the  earth's  centre  as  there 
is  on  the  other.  The  attractive  force, 
then,  of  the  earth  as  a  whole  is  ex- 
erted upon  the  body  in  the  direction 
of  this  middle  line.  It  tends  to  draw  it,  therefore,  towards 
the  centre.  Consequently,  a  plumb-line  points  towards  the 
centre  of  the  earth,  and  it  is  evident  that  two  weights  sus- 
pended by  two  strings  do  not  hang  per- 
fectly parallel  to  each  other.  The  dif- 
ference is  so  slight  in  an  ordinary  pair 
of  scales  that  it  cannot  be  perceived. 
But  if  it  were  possible  to  suspend  in 
the  heavens  a  beam  so  long  as  to  stretch 
over  a  large  extent  of  the  earth's  cir- 
cumference, as  represented  in  Fig.  11, 
the  scales  attached  to  it  would  be  very 
far  from  hanging  parallel  to  each  other. 
Substances  suspended  in  different  parts 
of  the  globe  are  hanging  in  different 
directions,  and  those  which  are  hung 


Fig.  10. 


ATTRACTIONS    OF   MATTER. 


55 


up  by  our  fellow-men  on  the  opposite  side  of  the  earth  are 
hanging  directly  towards  us. 

Up  and  Down. — All  falling  bodies  fall  towards  the  centre 
of  the  earth,  and  the  remarks  made  in  relation  to  suspend- 
ed weights  are  similarly  applicable.  Up  and  down  are 
merely  relative  terms — up  being  from  the  centre  of  the 
earth,  and  down  towards  it.  As  the  earth  moves  round  on 
its  axis,  the  same  line  of  direc- 
tion Avhich  we  call  upward  at 
one  time  is  downward  at  an- 
other. This  may  be  illustrated 
by  Fig.  12.  Let  the  circle  rep- 
resent the  circumference  of  the 
earth.  In  the  daily  revolution 
we  pass  over  this  whole  circle. 
If  we  are  at  D  at  noon,  we  are 
at  E  at  six  o'clock,  and  at  F 
at  midnight.  If,  therefore,  the 
ball  A  be  dropped  from  some 
height  at  noon,  the  line  in  which 
it  falls  will  be  at  right  angles 
to  a  line  in  which  it  will  fall  if  dropped  from  the  same 
height  at  six  o'clock;  for  this  height  will  have  moved  in 
this  same  time  from  A  to  B.  If  it  be  dropped  from-  the 
same  height  at  midnight,  its  line  of  direction  will  be  directly 
opposite  to  the  first ;  for  the  place  of  the  experiment  will 
have  moved  in  that  time  to  C. 

It  is  not  always  true  that  falling  bodies  tend  exactly  towards  the  centre  of 
the  earth.  The  centre  does  not  attract  them,  but  it  is  the  substance  of  the 
whole  earth ;  and  since  this  is  irregular  in  ijs  density  and  form,  the  attrac- 
tion will  be  irregular  also.  Thus  it  is  found  by  accurate  experiments  that 
a  plumb-line  suspended  in  the  neighborhood  of  a  mountain  is  attracted  by 
it,  and  will  not  hang  exactly  parallel  with  another  suspended  at  some  dis- 
tance from  the  mountain.  The  difference  is  not,  however,  enough  to  have 
any  practical  bearing. 


56  NATURAL  PHILOSOPHY. 

27.  "Weight. — That  which  we  call  weight  is  not  a  property 
of  matter,  but  merely  the  resisted  attraction  of  the  earth. 
If  two  bodies  fall  to  the  earth,  and  one  of  them  contain 
ten  times  as  many  particles  of  matter  as  the  other,  ten 
times  as  much  force  of  gravity  is  required,  and  is  actually 
exerted,  to  bring  it  to  the  ground.  This  will  appear  plain 
to  you  if  you  bear  in  mind  that  a  body  falls  because  it 
is  drawn  down  by  the  force  of  attraction,  and  then  com- 
pare this  force  to  any  other  force,  as,  for  example,  that  of 
muscular  action.  If  you  draw  towards  you  two  weights, 
one  of  which  is  twenty  times  as  heavy  as  the  other,  or,  in 
other  words,  has  twenty  times  as  great  a  quantity  of  mat- 
ter, you  must  exert  twenty  times  as  much  strength  on  the 
former  as  you  do  on  the  latter.  So  it  is  with  the  force  of 
attraction.  The  earth  attracts  a  body  having  twenty  times 
more  matter  than  another  with  twenty  times  the  amount 
of  force.  And  the  first  body  will  have  twenty  times  the 
weight  of  the  other,  for  it  will  make  twenty  times  the  press- 
ure upon  anything  that  resists  the  force  with  which  the  earth 
draws  it.  Weight,  then,  is  the  amount  of  resistance  to  the 
attraction  existing  beticeen  the  earth  and  the  body  weighed. 
If  you  place  a  substance  in  one  side  of  a  pair  of  scales,  it 
goes  down  because  of  the  attraction  between  it  and  the 
earth.  By  placing  weights  in  the  other  side  until  the 
scales  are  balanced,  you  find  how  much  is  needed  to  coun- 
teract the  resistance  caused  by  the  attraction  of  the  sub- 
stance and  the  earth  for  each  other ;  or,  in  other  words, 
you  find  out.  how  much  it  weighs.  In  doing  this  you  use 
certain  standard  weights;  that  is,  certain  quantities  of  mat- 
ter which  have  been  agreed  upon  by  mankind,  and  are 
called  by  certain  names,  as  ounces,  pounds,  grammes,  kilo- 
grammes, etc.  When  a  spring-balance  is  used,  the  spring 
has  been  tested  by  these  standard  weights,  and  its  scale 
marked  accordingly. 


ATTRACTIONS    OF   MATTER.  57 

Weight  not  Fixed,  but  Variable.  —  Weight  does  not  depend  alone 
upon  the  density  of  the  body  weighed,  but  also  upon  the  density  of  the 
earth.  For  the  attraction  causing  the  resistance  which  we  call  weight  is 
a  mutual  attraction,  and  is  in  proportion  to  the  quantities  of  matter  of 
both  the  body  and  the  earth.  If,  therefore,  the  density  of  the  earth  were 
increased  twice,  three  times,  or  four  times,  the  weights  of  all  bodies  would 
be  increased  in  the  same  proportion ;  that  is,  the  force  with  which  the 
earth  would  attract  them  would  be  twice,  three  times,  or  four  times  as 
great  as  now.  This  would  not  be  perceived  by  any  effect  on  balances, 
for  the  weights  and  the  articles  weighed  would  be  alike  increased  in 
weight.  But  it  would  be  perceived  in  instruments  that  indicate  the 
weights  of  bodies  by  their  influence  on  a  spring.  These  would  disagree 
with  scales  and  steelyards  just  in  proportion  to  the  increase  of  the  earth's 
density.  It  would  be  perceived  also  in  the  application  of  muscular  and 
other  forces  in  raising  and  sustaining  weights;  every  stone  would  require 
twice,  three  times,  or  four  times  the  muscular  effort  to  raise  it. 

28.  Weight  Varies  with  Distance.  —  The  nearer  two 
bodies  are  to  each  other,  the  greater  the  mutual  attrac- 
tion. The  nearer  a  body  is  to  the  earth,  the  greater 
the  attraction  that  draws  it  towards  the  earth — in  other 
words,  the  greater  is  its  weight.  The  force  of  gravity, 
or  Aveight,  is  greatest,  therefore,  just  at  the  surface  of  the 
earth,  and  it  diminishes  as  we  go  up  from  the  earth.  As 
we  leave  the  surface  of  the  earth,  the  force  of  gravity 
lessens  in  such  a  proportion  that  it  is  always  inversely  as 
the  square  of  the  distance  from  the  centre  of  the  earth.  In 
other  words,  the  force  of  gravity  increases  or  decreases  at 
the  square  of  the  rate  that  the  distance  decreases  or  in- 
creases. This  requires  still  further  explanation.  If  the 
distance  from  the  centre  of  the  earth  to  its  surface,  which 
is  4000  miles,  be  called  1,  then  4000  miles  from  the  earth 
would  be  called  2,  or  twice  as  far  from  the  centre,  and 
8000  miles  from  the  earth  would  be  3,  12,000  miles  from 
the  earth  would  be  4,  and  so  on.  The  squares  of  these 
numbers  are  1,  4,  9,  16,  etc.  Now,  since  weight  decreases 
inversely  as  the  square  of  the  distance,  any  object  weighing 


58  NATURAL   PHILOSOPHY. 

one  pound  on  the  surface  of  the  earth  would  weigh  but  -J 
pound  at  the  distance  of  4000  miles,  and  only  ^  pound  at 
8000  miles. 

An  object  weighs  less  on  the  summit  of  a  high  mountain 
than  in  the  valley  below,  because  it  is  farther  removed  from 
the  great  bulk  of  the  earth,  and  is  therefore  not  so  strongly 
attracted.  The  difference,  however,  is  but  small ;  a  man 
weighing  250  pounds  in  the  valley  would  weigh  but  half  a 
pound  less  on  the  summit  of  a  mountain  four  miles  high. 

We  have  spoken  of  weight  only  in  relation  to  the  earth,  but  weight  is  an 
attribute  of  bodies  everywhere,  for  wherever  matter  is  found  there  must  be 
attraction. 

The  weight  of  the  substances  on  the  surface  of  the  different  heavenly 
bodies  varies  according  to  the  quantity  of  matter  in,  or  density  of,  those 
bodies.  Since  the  moon  is  much  smaller  than  the  earth,  a  body  which 
weighs  a  pound  on  the  surface  of  the  earth  would  weigh  much  less  than  a 
pound  on  the  moon.  And  since  the  sun  is  much  larger  than  the  earth,  the 
same  body  carried  there  would  weigh  much  more  than  a  pound. 

If  we  knew  the  exact  densities  of  the  sun  and  the  moon  and  the  earth, 
ns  well  as  their  size,  we  could  estimate  exactly  the  difference  in  the 
weights  which  any  body  would  have  in  them;  for  the  attraction  which 
causes  the  resistance  called  weight  is  in  direct  proportion  to  the  quantity 
of  matter,  and  the  quantity  of  matter  depends  on  both  density  and  size. 

29.  Cohesion. — That  form  of  attraction  which  binds  to- 
gether the  molecules  of  a  body  is  called  cohesion. 

Cohesion  is  stronger  in  some  solids  than  in  others.  The 
mason  with  his  trowel  easily  divides  a  brick;  but  he  can- 
not do  this  with  a  piece  of  granite,  for  its  particles  have  a 
greater  attraction  for  each  other  than  those  of  the  brick. 
A  blow  which  would  break  a  glass  dish  would  not  in- 
jure a  copper  one  of  the  same  thickness.  A  weight  that 
would  hang  securely  from  an  iron  wire  would  break  a  lead 
wire  of  the  same  size;  that  is,  it  would  tear  the  particles 
apart,  because  they  are  not  strongly  attracted  to  each 
other.  Cohesion  has  different  modes  of  action  in  different 


ATTRACTIONS    OP    MATTER.  59 

solids.  It  therefore  fastens  their  particles  together  in  dif- 
ferent ways,  and  thus  occasions  the  physical  properties 
which  are  so  useful  to  us — tenacity,  elasticity,  ductility, 
flexibility,  etc. 

Cohesion  is  exerted  only  between  molecules  of  the  same 
kind;  when  two  masses  are  made  to  cohere  they  must  be 
of  similar  material  and  must  be  pressed  very  closely  to- 
gether, because  the  attractive  force  is  exerted  only  at  in- 
appreciable distances.  For  this  same  reason  also  it  is  only 
the  surface  particles  which  influence  the  cohesion. 

Examples  of  cohesion  of  masses  are  numerous :  two 
highly  polished  surfaces  of  glass  may  be  made  to  stick 
together  as  if  glued,  and  can  only  be  separated  by  slid- 
ing one  off  the  other. 

Before  rubber  tubing  was  a  commercial  article,  it  was  made  by  a  simple 
process  based  upon  its  cohesive  property.  A  piece  of  sheet  rubber  of 
suitable  length  and  width  is  wrapped  around  a  glass  or  wooden  rod,  and 
a  strip  cut  off,  where  the  edges  lap,  with  a  pair  of  long  scissors ;  by  press- 
ing together  the  freshly  cut  surfaces,  they  cohere  firmly,  making  a  perfect 
tube. 

The  manufacture  of  various  articles  which  are  made  by  compressing 
powders  until  they  form  solids,  as  in  the  case  of  graphite  for  lead-pencils, 
sawdust  for  wooden  ornaments,  brick-dust  for  tiles,  etc.,  are  examples 
of  practical  applications  of  cohesion. 

If  you  cut  two  bullets  so  as  to  give  to  each  a  very  smooth  flat  sur- 
face, you  can  make  them  cohere  quite  strongly  by  pressing  them  to- 
gether, especially  if  yon  give  a  little  turning  motion  at  the  same  time  that 
you  press,  for  this  will  bring  the  particles  on  the  surfaces  in  close  con- 
tact. If  the  balls  of  lead  are  quite  large  and  furnished  with  handles,  as 

represented  in  Fig.  13,  it  will  require 
considerable   force   to   separate   them 
when  they  have  been  thus  pressed  to- 
Fig.  13.  gether. 

30.  Cohesion  in  Liquids. — In  liquids  the  attraction  be- 
tween the  particles  is  very  feeble  compared  witli  that  in 
solids.  The  strength  of  the  attraction  of  particles  of  steel  is 


CO  NATUKAL   PHILOSOPHY. 

about  three  million  times  that  of  the  particles  of  water. 
The  estimate  is  made  in  this  way:  We  find  that  a  stool 
wire  will  sustain  a  weight  equal  to  39,000  feet  (11.887  kilo- 
metres) of  the  wire.  But  a  drop  of  water  hanging  to  the 
end  of  a  stick  cannot  be  more  than  one  sixth  of  an  inch 
(42  millimetres)  in  length ;  that  is,  water  will  hold  together 
by  the  attraction  of  its  particles  only  to  this  extent,  which 
is  a  little  less  than  the  three-millionth  part  of  the  length 
of  steel  wire  which  could  hang  without  breaking. 

The  freedom  with  which  the  particles  of  a  liquid  move 
among  one  another  is  due  to  the  comparative  feebleness 
of  cohesion,  and  to  the  fact  that  the  molecules  are  more 
widely  separated  than  in  solids.  This  mobility  of  liquids 
varies  considerably  according  to  the  intensity  of  the  co- 
hesive power;  in  limpid  liquids,  such  as  ether,  alcohol, 
naphtha,  etc.,  the  force  of  cohesion  is  very  feeble,  while  viscid 
liquids,  such  as  oil,  molasses,  glycerin,  etc.,  are  sluggish  in 
their  motions,  being  hampered  by  greater  cohesion  of  their 
particles.  For  this  reason,  too,  drops  of  viscous  liquids  are 
much  larger  than  those  of  mobile  ones  poured  from  the 
same  bottle ;  sixty  drops  of  water  fill  the  same  measure  as 
one  hundred  of  laudanum  when  poured  from  a  lip  of  the 
same  size.  A  knowledge  of  this  and  similar  facts  is  of  im- 
portance to  physicians  and  druggists. 

31.  Globular  Shape  of  Drops  of  Liquid. — Since  the  parti- 
cles of  a  liquid  move  thus  freely  among  each  other,  the 
attraction  of  cohesion  disposes  them  to 
assume  a  globular  or  spherical  shape. 
The  reason  of  this  can  be  made  plain 
by  Figs.  14  and  15.    The  outside  of  a 
perfect  sphere  is  all  at  the  same  dis- 
tance from  the  centre ;  and  the  circum- 
ference of  a  circle  is  equidistant  from 
Fig.  14.  the  centre,  as  represented  in  Fig.  14. 


ATTRACTIONS   OF  MATTER.  61 

* 

But  this  is  not  true  of  all  parts  of  the  surface  of  a  cube  or 
of  a  square :  a,  for  example,  is  farther  from  the  centre  than 
b.  Now  in  a  drop  of  liquid  all  the  particles  are  attracted 
towards  the  centre,  for  in  that  line  from  each  particle  lies 
the  largest  number  of  particles  to  attract  it.  This  can  be 
made  obvious  by  taking  some  point  in  the  drop,  as  repre- 
sented in  Fig.  15,  and  drawing  lines 
from  it  through  the  centre  and  in 
other  directions.  If  a  be  the  point 
in  the  drop,  it  is  plain  that  the  line 
from  it  through  the  centre  is  longer 
than  a  b  or  a  c.  Therefore  a  parti- 
cle, a,  will  be  attracted  towards  the 
centre  rather  than  in  the  direction  Fig.  i&. 

ab  ov  a  c,  because  there  are  more  particles  in  the  direction 
of  the  centre,  and  the  more  particles  there  are  the  stronger 
is  the  attraction.  But  this  is  not  all.  The  particles  in  the 
line  a  c,  tending  to  make  a  go  towards  c,  are  balanced  by 
the  particles  in  the  line  a  e,  tending  to  make  it  go  towards 
e.  The  two  lines  of  particles,  therefore,  together  tend  to 
make  it  go  in  a  middle  line  between  them ;  that  is,  to- 
wards the  centre,  just  as  two  strings  pulling  equally,  the 
one  to  c  and  the  other  to  e,  would  make  a  body,  a,  move 
in  a  middle  line  between  these  two  directions.  The  same 
can  be  shown  of  the  two  lines  of  particles  a  b  and  a  d, 
and  so  of  any  other  two  alike  in  situation  on  each  side  of 
the  line  through  the  centre.  The  tendency  of  every  par- 
ticle is,  then,  to  move  towards  the  centre,  and  a  globular 
form  results. 

32.  The  Spherical  Form  in  Different  Liquids. — The  dis- 
position to  form  a  sphere  is  seen  more  distinctly  in  mercury 
than  in  any  other  liquid.  If  you  drop  a  little  of  it  upon  a 
plate  it  separates  into  globules,  which  roll  about  like  shot. 
Why  does  water  behave  differently  ?  Why  do  the  drops 

C2 


G2  NATURAL   PHILOSOPHY. 

of  water  hang  upon  the  window-pane,  showing  only  in 
an  imperfect  way  their  disposition  to  a  globular  arrange- 
ment ?  It  is  because  the  particles  of  water  have  a  greater 
attraction  for  other  substances,  and  less  attraction  for  each 
other,  than  the  particles  of  quicksilver.  Water  sometimes 
exhibits  its  disposition  to  form  spheroidal  drops  on  the 
leaves  of  some  plants,  and  rolls  about  in  balls  like  mercury. 
This  is  because  there  is  something  on  the  surface  of  the 
leaf  which  repels  rather  than  attracts  the  water.  If  you 
put  your  finger,  however,  on  one  of  these  drops,  it  will  alter 
its  shape,  and  your  finger  will  be  moistened,  because  there 
is  an  attraction  between  the  particles  of  your  skin  and 
those  of  the  water.  Take  another  illustration  of  this  dif- 
ference in  attraction.  If  you  drop  a  little  oil  upon  the 
surface  of  water  it  will  float  about  in  round  drops.  This 
is  because  the  water  repels  the  oil.  But  when  oil  is  spilled 
upon  wood  or  cloth  their  particles  have  so  strong  an  at- 
traction that  they  unite,  instead  of  gathering  into  little 
round  masses  as  they  do  on  the  surface  of  water. 

Manufacture  of  Shot. — We  have  a  good  example  of  the  tendency  of 
fluids  to  form  spherical  drops  in  the  manufacture  of  shot.  Melted  lead 
is  poured  into  a  large  vessel,  in  the  top  of  the  shot-tower,  having  holes  in 
its  bottom,  from  which  the  metal  falls  in  drops.  Each  drop,  as  it  whirls 
round  and  round  in  its  fall,  takes  the  globular  form.  By  the  time  that  it 
reaches  the  end  of  its  journey,  about  two  hundred  feet,  it  becomes  so  far 
cooled  as  to  be  solid,  and  as  it  is  received  in  a  reservoir  of  water,  its  glob- 
ular form  is  retained.  Bullets  cannot  be  made  in  this  way,  because  a 
quantity  of  melted  lead  sufficient  to  make  a  bullet  will  not  hold  together 
in  a  globular  form. 

33.  Spherical  Form  of  the  Earth  and  the  Heavenly  Bodies. 
— It  is  supposed  that  the  sun,  moon,  earth,  and  all  the  heav- 
enly bodies  were  once  in  a  liquid  state,  and  that  they  owe 
their  spherical  shape  to  this  fact.  As  they  whirled  on 
in  their  course,  the  liquid  mass  gradually  cooled,  and  at 
length  they  acquired  their  present  state.  How  all  the 


ATTRACTIONS    OP   MATTER.  63 

mighty  changes  could  be  effected  in  our  earth,  converting 
it  from  a  liquid  into  a  body  with  a  solid  crust,  having  such 
a  diversity  of  substances  in  it,  and  so  variously  arranged, 
with  its  depressions  containing  water,  and  the  whole  cov- 
ered with  its  robe  of  air  fifty  miles  in  thickness,  we  cannot 
fully  understand.  And  yet  there  are  some  portions  of  the 
process  which  chemistry  and  geology  have  revealed  to  us, 
giving  us  some  glimpses  of  the  wonders  which,  during  the 
lapse  of  ages,  God  wrought  in  our  earth  in  preparing  it 
for  the  habitation  of  man. 

34.  Crystallization. — The  attraction  of  cohesion  is  not  in 
all  cases  uniformly  strong  in  all  directions  around  a  mole- 
cule, and  when  the  particles  are  free  to  move  they  often 
assume  a  more  or  less  regular  arrangement,  becoming  crys- 
talline. This  happens  most  frequently  when  a  substance 
passes  from  a  liquid  state  to  a  solid  one,  and  when  it  is  de- 
posited from  a  solution. 

The  process  of  crystallization  is  readily  studied  by  slow- 
ly cooling  saturated  solutions  of  certain  chemical  sub- 
stances: alum,  saltpeter,  sulphate  of  copper,  borax,  and 
other  substances.  There  is  an  immense  variety  of  crys- 
talline forms,  the  study  of  which  is  pursued  in  connection 
with  the  science  of  mineralogy;  we 
can  here  merely  indicate  a  few  of  the 
forms  which  substances  assume.  Com- 
mon salt  crystallizes  in  cubes,  Fig.  16; 
alum  in  octahe- 
dra, or  eight-sided 
figures,  Fig.  17. 
Crystalline  forms 

are  also  assumed  by  many  minerals: 
the  bright-red  garnet  crystallizes  in 
the  form  shown  in  Fig.  18,  and  quartz 
takes  the  form  of  Fi.  19.  All  the 


64 


NATURAL  PHILOSOPHY. 


Fig.  19. 


precious  stones  have  a  crys- 
talline structure,  and  even 
the  common  rocks  under  your 
feet  exhibit  the  same  crystal- 
line disposition  in  detail  which 
you  see  in  the  mass. 

Pig. is.  Water,   when  it    changes 

into  a  solid,  shows  the  same  disposition,  of 
which  the  crystals  of  the  snow  and  the  frost- 
work on  our  windows  are  familiar  examples. 
When  snow  forms,  the  water  of  the  clouds  is  suddenly 
crystallized  by  the  cold  air,  the  particles  taking  their 
regular  places  more  readily  and  certainly  than  if  they 
were  guided  by  intelligence,  because  in  obedience  to  an 
unerring  law  established  by  the  Creator.  Examples  of 
this  sudden  crystallization  of  water  are  common.  The 
water  in  a  pitcher  may  remain  fluid,  although  it  is  cooled 
down  to  the  freezing-point,  and  even  below  it,  if  it  be  kept 
perfectly  still.  But  on  agitating  the  pitcher  the  water  at 
once  becomes  filled  with  a  net-work  of  ice-crystals.  The 
stillness  of  the  water  prevented  its  particles  from  taking 
the  crystalline  arrangement  needed  for  the  formation  of 
ice  ;  and  the  shaking  of  the  particles  assisted  the  motion 
necessary  to  the  assumption  of  a  crystalline  form. 

35.  Frost  and  Snow. — The  frost-work  on  our  windows  is 
a  wonderful  exhibition  of  the  variety  of  forms  that  crystal- 
lization can  produce.  It  sometimes  presents  figures  like 
leaves  and  flowers,  such  as  are  chased  on  vessels  of  silver, 
but  much  more  delicate  and  beautiful.  So  varied  and  fan- 
tastic are  the  forms  in  which  these  water-crystals  are  ar- 
ranged, that  it  is  very  natural  to  ascribe  them,  as  is  done 
universally  in  the  dialect  of  the  nursery,  to  the  ingenuity 
of  a  strange  and  fanciful  spirit.  Every  snow-flake  is  a 
bundle  of  little  crystals  as  regular  and  beautiful  as  the 


ATTRACTIONS    OF   MATTER. 


C5 


crystals  which  you  so  much  ad- 
mire in  a  inineralogical  cabinet. 
And  there  is  great  variety  in  the 
grouping  of  these  crystals.  Figs. 
20  and  21  show  some  of  these 
forms  as  they  appear  under  the 
microscope. 

It  is  a  very  quick  operation  by  which  the 
particles  of  water  in  the  clouds  thus  mar- 
shal themselves,  as  if  by  magic,  in  these 
regular  forms.  But  a  quicker  operation 
is  that  by  which  hail  is  formed  —  so  quick 
that  the  particles  have  not  time  to  arrange 
themselves  in  crystalline  forms,  but  are  huddled  together  without  order. 
The  brilliant  and  glistening  whiteness  of  the  snow  is  owing  to  the  reflec- 
tion of  light  from  its  minute  crystals.  In  the  arctic  regions  the  beauty 
of  the  snow  is  often  much  greater  than  with  us.  "The  snow  crystals  of 
last  night,"  says  Captain  M'Clintock  in  his  "Discovery  of  the  Fate  of  Sir 
John  Franklin,"  "were  extremely  beautiful.  The  largest  kind  is  an  inch 
in  length  ;  its  form  exactly  resembles  the  end  of  a  pointed  feather.  Stellar 

crystals  two  tenths  of  an  inch 
in  diameter  have  also  fallen  ; 
these  have  six  points,  and 
are  the  most  exquisite  things 
when  seen  under  a  micro- 
scope. In  the  sun,  or  even 
in  moonlight,  all  these  crys- 
tals" glisten  most  brilliantly; 
and  as  our  masts  and  rig- 
ging are  abundantly  covered 
with  them,  the  Fox  never 
was  so  gorgeously  arrayed  as 
she  now  appears." 

Order  in  Nature.  — 
We  see  in  this  gen- 
eral tendency  to  crys- 


tallization 


a    striking 


66  NATURAL  PHILOSOPHY. 

illustration  of  the  fact  that  the  Almighty  is  a  God  of  order. 
Disorderly  arrangement  is  never  seen  except  where  there 
is  an  obvious  necessity  for  it.  And  even  when  there  is  ap- 
parent disorder,  a  little  examination  generally  shows  that 
essentially  there  is  order.  The  rocks  that  give  so  much 
variety  to  scenery  seem  to  be  piled  up  in  confusion,  yet  or- 
der has  evidently  reigned  in  their  construction.  Pick  up  a 
common  stone,  and  on  breaking  it  you  will  see  the  crystal- 
line arrangement  in  its  interior.  Nay,  more,  much  of  the 
very  soil  is  made  up  of  separated  and  broken  crystals. 

Amorphous  Bodies. — Substances  which  possess  no  regu- 
larity of  structure  are  termed  amorphous,  that  is,  without 
crystalline  form.  Glue,  soap,  clay,  chalk,  and  many  min- 
erals are  amorphous.  Some  substances  may  occur  at  one 
time  in  a  crystalline  state  and  at  another  without  any  trace 
of  regularity  of  form.  Carbonate  of  lime  is  one  of  these, 
being  crystalline  in  limestone,  Iceland  spar,  and  various 
minerals,  while  in  the  form  of  chalk  it  is  amorphous.  Met- 
als, too,  may  occur  both  amorphous  and  crystalline.  The 
attraction  of  cohesion,  which  produces  crystalline  forms, 
leads  to  peculiarities  of  structure  which  receive  special 
names,  as  "hard,"  "brittle,"  etc.,  as  already  explained  in 
Chapter  III. 

QUESTIONS. 

24.  What  is  said  of  the  attraction  of  matter  ?  What  is  the  force  gener- 
ally called,  and  why  ?  Show  that  attraction  exists  between  small  masses. 
Name  and  define  the  different  kinds  of  attraction. — 25.  What  is  said  of 
gravitation  ?  Illustrate  the  fact  that  attraction  is  mutual.  Illustrate  also 
the  proportions  of  the  mutual  attractions.  What  is  said  of  the  motion  of 
the  earth?  —  20.  Explain  why  bodies  are  attracted  towards  the  earth's 
centre.  How  does  this  affect  plumb-lines  suspended  at  some  distance  from 
one  another?  Show  that  up  and  down  are  only  relative  terms.  Why  do 
falling  bodies  deviate  from  a  line  drawn  exactly  to  the  earth's  centre? — 
27.  What  is  weight?  Give  the  comparison  in  regard  to  muscular  force. 
What  is  said  of  scales  and  weights?  What  of  using  springs  in  weighing? 


ATTRACTIONS    OF   MATTER.  67 

• — 28.  What  would  be  the  effect  on  weight  if  the  density  of  the  earth  were 
increased  ?  In  what  ways  would  this  be  perceived  ? — 29.  What  is  said  of 
the  variation  of  weight  with  distance?  Explain  the  law.  What  is  said  of 
the  difference  of  weight  on  mountains  and  in  valleys  ?  What  is  said  of 
the  weight  of  bodies  on  the  moon? — 30.  What  is  said  of  cohesion?  Give 
examples  of  cohesion.  Describe  the  experiment  with  two  bullets. — 31. 
What  is  said  of  cohesion  in  liquids?  What  of  the  mobility  of  liquids? 
What  causes  some  liquids  to  be  limpid  and  some  viscid?  Explain  by 
reference  to  Figs.  14  and  15  the  globular  form  of  drops. — 32.  Give  the 
difference  between  mercury  and  water  in  regard  to  the  spherical  form. 
What  is  said  of  drops  of  water  on  leaves  ?  What  is  said  of  oil  in  ref- 
erence to  attraction?  Describe  and  explain  the  manufacture  of  shot. — 
33.  What  is  said  of  the  spherical  form  of  the  earth  and  the  heavenly 
bodies? — 34.  What  is  said  of  crystallization  ?  State  the  examples  cited. 
What  is  said  of  the  crystallization  of  water?  Give  and  explain  the  ex- 
ample of  sudden  crystallization. — 35.  What  is  said  of  frost-work  ?  What 
of  snow?  What  is  stated  in  regard  to  the  snow -crystals  of  the  arctic 
regions?  What  is  said  of  order  in  nature?  What  of  amorphous  bodies? 


CHAPTER  V. 

ATTRACTIONS    OF   MATTER    (CONTINUED). 

36.  Adhesion. — Adhesion  is  the  attraction  between  dif- 
ferent kinds  of  matter,  as  between  solids  and  liquids,  or 
between  glue  and  wood.  When  a  glass  article  is  broken 
you  cannot  unite  the  pieces,  however  accurately  you  may 
bring  them  together,  or  however  firmly  they  may  be 
pressed.  This  is  because  the  power  of  cohesion  acts  strong- 
ly only  when  the  molecules  are  brought  very  near  to- 
gether; and  it  is  impossible  to  bring  the  particles  on  the 
two  surfaces  of  a  broken  piece  of  glass  as  near  together  as 
they  were  before  the  fracture.  If  it  were  possible  so  to  do, 
no  crack  would  be  visible.  We  are  obliged  therefore  to 
resort  to  some  kind  of  cement;  this  causes  the  two  surfaces 


68  NATUKAL  PHILOSOPHY. 

to  adhere  because,  while  soft,  it  insinuates  itself  among  the 
particles  of  glass,  and  on  drying  becomes  a  bond  of  union 
between  the  broken  fragments.  In  adhesion  as  well  as  in 
cohesion  only  the  surface  layer  of  molecules  exert  any  in- 
fluence, consequently  a  mere  film  over  a  surface  suffices  to 
alter  its  adhesive  power,  as  when  a  glass  is  greasy. 

Examples  of  the  adhesion  of  solids  are  familiar:  silver 
and  gold  may  be  made  to  adhere  to  iron  by  a  very  great 
and  sudden  pressure.  The  iron  must  be  made  very  smooth, 
and  the  silver  or  gold  plate  very  thin.  A  powerful  blow 
brings  the  particles  of  the  thin  plate  into  such  nearness  to 
those  of  the  iron  that  union  is  affected,  or,  in  other  words, 
they  attract  each  other  sufficiently  to  be  united.  Similarly, 
a  sheet  of  tin  and  one  of  lead  can  be  made  to  adhere  so  as 
to  form  one  sheet  by  the  pressure  of  the  rollers  of  a  roll- 
ing-mill. 

37.  Adhesion  of  Solids  and  Liquids. — The  attraction  which 
solids  and  liquids  have  for  each  other  furnishes  us  with 
many  interesting  phenomena.  The  adhesion  of  drops  of 
water  to  glass  and  other  solids  is  a  familiar  example  of  this 
attraction.  If  you  dip  your  hand  into  water,  it  is  wet  on 
taking  it  out,  because  your  skin  has  sufficient  attraction  for 
the  water  to  retain  some  of  it.  A  towel  will  retain  more 
of  it  for  two  reasons :  owing  to  the  interstices  between  its 
fibres  it  presents  much  more  surface  to  the  water  (see  §  39, 
Capillary  Attraction),  and  it  has  none  of  the  oily  substance 
which  on  your  skin,  though  in  small  quantity,  serves  some- 
what to  repel  the  water. 

If  you  clip  your  hand  into  mercury  the  latter  will  not 
adhere  to  it,  and  it  would  seem  that  the  skin  has  an  at- 
traction for  water  and  none  for  mercury.  This,  however, 
is  only  apparent,  for  a  small  globule  of  mercury  will  adhere 
to  the  finger,  though  if  it  be  brought  in  contact  with  a 
larger  amount  of  mercury  the  globule  leaves  the  finger  and 


ATTRACTIONS    OF   MATTER. 


C9 


loses  itself  in  the  liquid.  This  shows  that  liquids  wet  solids 
when  the  adhesion  of  the  liquid  to  the  solid  is  greater  than 
the  cohesion  of  the  liquid. 

The  attraction  of  solids  and  fluids  for  each  other  is  shown 
very  prettily  in  the  experiment  represented  in  Fig.  22.  A 
plate  of  glass  is  attached  by  strings  to  one  end  of  a  bal- 
ance, and  weights  just  sufficient  to  balance  it  are  placed  in 


r  4 

U" "• '  ••!'":  '-'-'-'    '  •''""•  "••  •'!"'!'!•••'•   '•  y'~''~       ''\'-'i' -1  •"' 'Tvr"iM^:.,«ar-a| 

Fig.  22. 

the  opposite  scale.  When  the  glass  is  brought  in  contact 
with  water,  it  will  require  additional  weight  in  the  scale  to 
separate  the  glass  from  the  water.  This  experiment,  how- 
ever, does  not  measure  the  adhesion  of  the  glass  and  water 
accurately,  because  we  cannot  detach  the  plate  of  glass 
clean  and  dry ;  it  merely  measures  the  'force  necessary  to 
overcome  the  cohesion  of  the  liquid. 

Further  Illustrations. — When  you  see  stems  of 
plants  rising  above  the  surface  of  stagnant  water 
you  will  observe  that  the  water  is  considerably 
raised  about  them.    This  is  from  the  attraction  be- 
tween them  and  the  water.     For  the  same  rea- 
son water  rises  higher  at  the  sides  of  a  tumbler  Fig. 23- 
than  in  the  middle.      If  you  immerse  a  piece  of  glass  in  water,  the 
water  will  rise  at  its  sides  as  represented  in  Fig.  23.     If  you  immerse  two 


NATURAL   PHILOSOPHY. 


pieces  together,  as  in  Fig.  24,  the  water  will  rise  higher  between  them  than 
on  the  outside,  because  the  particles  between  are  attracted  by  two  surfaces, 
while  those  outside  are  attracted  by  only  one.  It  is  for  the  same  reason 


Fig.  24.  Fig.  -25. 

that  two  men  can  raise  a  weight  higher  than  one  of  them  can  alone.  And 
if  the  pieces  of  glass  be  brought  quite  near  together,  as  in  Fig.  25,  the 
water  will  be  raised  higher  still,  because  there  is  less  to  be  raised  by  the 
two  surfaces.  Just  as  two  men  can  raise  a  small  weight  higher  than  they 

can  a  large  one.  The  same 
thing  may  be  beautifully  illus- 
trated in  this  way :  Let  two 
pieces  of  glass,  as  represented 
in  Fig.  2G,  be  immersed  in  col- 
ored water,  with  two  of  their 
edges  joined  together,  the  op- 
posite edges  being  separated. 
The  height  to  which  the  fluid 
rises  will  make  a  curved  line,  it 
being  lowest  at  the  edges  which 
are  separated,  and  highest  at 


Fig.  26. 


the  edges  which  are  joined. 


38.  Rise  of  Liquids  in  Tubes. — For  the  same  reason  that 
water  rises  higher  between  plates  of  glass  than  outside,  it 
will  rise  higher  within  a  tube  than  on  the  outside.  The 
diagram  in  Fig.  27  will  make  this  clear.  It  represents  a 
transverse  section  of  a  tube,  enlarged  so  that  the  demon- 
stration may  be  plain.  Consider  the  case  of  one  particle 
on  the  inside  and  another  on  the  outside  at  equal  distances 
from  the  glass.  It  is  evident  that  the  particle  a  is  not  so 
near  to  as  many  particles  of  the  glass  as  is  the  particle  b. 


ATTRACTIONS    OF   MATTER. 


The  lines  drawn  show  this.  The  longest  lines  extending 
from  the  particles  a  and  b  to  the 
glass  are  equal  in  length ;  that  is, 
a  e  and  a  f  are  equal  to  b  g  and 
b  h.  It  is  clear,  therefore,  that  all 
the  glass  between  the  lines  at  c 
and  d  is  as  near  to  the  particle  b 
as  the  glass  between  the  lines  at 
e  and /'is  to  the  particle  a.  But 
this  is  not  all.  The  particle  b  is 
near  enough  to  the  whole  inside  of  the  tube  to  be  attracted 
by  it,  while  very  little  attraction  is  exerted  upon  a  by  any 
part  of  the  glass  beyond  that  which  is  included  between  e 
and  /.  The  same  difference  can  be  shown  with  regard  to 
all  the  particles  on  the  inside  of  the  tube  compared  with 
those  outside.  The  former  are  nearer  to  more  particles  of 
the  glass  than  the  latter,  and  therefore  are  more  strongly 
attracted.  Again,  the  nearer  the 
plates  of  glass,  the  higher  the  water 
rises  between  them ;  so  the  smaller 
the  tube,  the  higher  will  the  water 
rise  in  it.  You  can  try  the  experi- 
ment by  immersing  in  water  glass 
tubes  of  different  diameters,  as  rep- 
resented in  Fig.  28.  It  is  obvious  that  the  particle  b  (Fig. 
27)  would  not  be  very  strongly  attracted  by  the  part  of 
the  tube  opposite  if  the  tube  were  a  large  one ;  but  it 
would  be  if  the  tube  were  very  small, 
for  then  it  would  be  quite  near  to  that 
part.  Since  glass  is  not  wet  by  mer- 
cury, a  tube  plunged  into  this  liquid 
causes  a  depression  without  and  with- 
in. Figs.  29  and  30  show  the  contrast 
between  water  and  mercury.  Fi 


72  NATURAL   PHILOSOPHY. 

39.  Capillary  Attraction.  —  The  term  capillary  (derived 
from  the  Latin  word  capilla^  hair)  has  been  commonly  ap- 
plied to  the  attraction  exhibited  under  the  circumstances 
just  noticed,  because  it  is  most  obvious  and  was  first  ob- 
served in  tubes  of  very  fine  bore.  The  same  term  is  used 
when  the  attraction  is  seen  in  the  rising  or  spreading  of  a 
liquid  in  interstices  as  well  as  in  tubes.  Thus  capillary 
attraction  causes  the  rising  of  oil  or  burning-fluid  in  the 
wicks  of  lamps.  The  liquid  ascends  in  the  interstices,  or 
spaces,  between  the  fibres,  as  it  does  in  the  spaces  of  tubes. 

Other  Examples. — If  you  let  one  end  of  a  towel  lie  in  a  bowl  of  water, 
the  other  end  lying  over  upon  the  table,  the  whole  towel  will  become  wet 
from  the  spreading  of  the  water  among  the  fibres  in  obedience  to  capillary 
attraction.  If  you  suspend  a  piece  of  sponge  so  that  it  merely  touch  the 
surface  of  some  water,  or  if  you  lay  it  in  a  plate  with  water  in  it,  the 
whole  sponge  will  become  wet.  So,  too,  if  you  dip  the  end  of  a  lump  of 
sugar  in  your  tea,  and  hold  it  there  a  little  time,  the  whole  lump  will  be 
moistened.  In  very  damp  weather  the  wood-work  in  our  houses  swells 
from  the  spreading  of  water  in  the  pores  of  the  wood  in  obedience  to 
capillary  attraction.  Especially  is  this  the  case  in  basement  rooms, 
where  the  water  can  ascend  from,  the  ground  in  the  pores  of  the  walls, 
as  well  as  from  the  damp  air.  In  watering  plants  in  pots,  if  the  water 
be  poured  into  the  saucers,  it  .will  pass  through  the  earth  by  capillary 
attraction.  For  the  same  reason  plants  and  trees  near  streams  grow 
luxuriantly,  being  abundantly  supplied  with  water,  which  rises  to  their 
roots  through  the  pores  of  the  soil.  The  disposition  of  the  wood  to  imbibe 
moisture  in  its  pores  has  sometimes  been  made  use  of  very  effectually  in 
quarrying  out  millstones.  First  a  large  block  of  stone  is  hewn  into  a 
cylindrical  shape.  Then  grooves  are  cut  into  it  all  around  where  a  sepa- 
ration is  desired,  and  wooden  wedges  are  driven  tightly  into  them.  These 
are  then  moistened  with  water,  and  eventually  swell  so  much  as  to  split 
the  stone  in  the  direction  of  the  grooves.  Blotting-paper  furnishes  an 
illustration  of  capillary  attraction,  the  ink  being  taken  up  among  the  fibres 
of  the  paper.  Ordinary  writing-paper  will  not  answer  as  a  blotter,  because 
the  sizing  fills  up  the  interstices  between  the  fibres. 

As  already  stated  (§  38),  whenever  a  body  is  wet  by  a 
liquid,  a  rise  of  its  surface  ensues ;  but  when  otherwise, 


ATTRACTIONS    OF   MATTER. 


a  depression  takes  place.  Thus  a  sewing-needle  washed 
with  alcohol  is  easily  wet  by  water  when  placed  on  its 
surface,  and  sinks  immediately ;  when,  however,  the  same 
needle  is  somewhat  greasy,  so  that  it  can  make  a  depres- 
sion, it  will  float.  Some  insects  which  skip  about  on  the 


Fig.  31. 

surface  of  the  water  are  protected  from  being  wet  by  it. 
The  feathers  of  water-fowl  are  always  slightly  oily,  and  thus 
they  remain  quite  dry  even  when  swimming  in  the  water. 

40.  Opposition  between  the  Modes  of  Attraction.  —  Al- 
though adhesion  and  gravitation  are  essentially  the  same 
thing,  we  see  them  continually  acting  in  opposition  to 
each  other.  Abundant  illustrations  might  be  given,  but 
we  will  cite  only  a  few. 

If  you  pour  water  out  of  a  tumbler,  there  is  a  struggle 
between  the  attraction  of  adhesion  and  gravitation  for  the 
mastery — the  attraction  of  adhesion  tending  to  make  the 
water  adhere  to  the  tumbler,  and  run  down  its  side,  as  in 
Fig.  32,  and  gravita- 
tion tending  to  make 
it  fall  straight  down. 
But  when  water  is 
poured  out  of  a  pitch- 
er, as  in  Fig.  33,  the  ( 
lip  of  the  pitcher  acts 
in  favor  of  the  at-  Fig.  32.  Fig. 


74  NATURAL  PHILOSOPHY. 

traction  of  gravity;  for  the  water  would  have  to  turn  a 
very  sharp  corner  to  run  down  the  outside  of  the  pitcher 
iu  obedience  to  adhesion.  In  pouring  water  from  a  tum- 
bler, we  can  often,  by  a  quick  movement,  throw  the  water, 
as  we  may  say,  into  the  hands  of  gravity  before  the  attrac- 
tion of  adhesion  can  get  a  chance  to  turn  it  down  the 
tumbler's  side.  If  you  can  only  make  the  water  begin 
to  run  from  the  tumbler  without  going  down  its  side 
there  will  be  no  difficulty;  for  there  is  an  attraction  of 
cohesion  between  the  particles  of  the  water,  tending  to 
make  them  keep  together,  which  in  this  case  acts  against 
the  adhesion  between  the  water  and  the  glass,  and  there- 
fore acts  in  favor  of  gravitation.  It  is  adhesion  together 
with  cohesion  that  forms  the  drop  on  the  lip  of  a  bottle 
as  we  drop  medicine— cohesion  between  the  particles  of 
the  liquid,  and  adhesion  between  the  latter  particles  and 
those  of  the  glass.  It  is  gravitation,  on  the  other  hand, 
that  makes  the  drop  fall,  it  becoming  so  large  that  the 
force  of  gravity  overcomes  the  adhesion  between  the  drop 
and  the  bottle. 

Size  of  Drops  Influenced  by  Gravitation. — Were  it  not  for  the  attraction 
of  gravitation,  there  would  be  no  limit  to  the  size  of  drops  of  any  liquid. 
When  the  drop  reaches  a  certain  size,  it  falls  because  it  is  so  heavy ;  or, 
in  other  words,  because  with  its  slight  adhesion  the  attraction  of  the  earth 
brings  it  down.  Now  if  this  attraction  could  be  suspended,  and  the  at- 
traction of  adhesion  left  to  act  alone,  particles  of  water  might  be  added  to 
the  drop  to  any  extent,  and  they  would  cling  there.  You  can  see  the 
struggle  between  adhesion  and  gravitation  very  prettily  illustrated  if  you 
watch  the  drops  of  rain  on  a  window-pane.  If  two  drops  happen  to  be 
quite  near  together,  they  unite  by  attraction,  and  then,  being  too  large  to 
allow  of  its  being  retained  there  by  adhesion  in  opposition  to  gravitation, 
the  united  drop  runs  down.  If  it  meet  with  no  other  drop,  it  soon  stops, 
because  by  adhesion  some  portion  of  it  clings  to  the  glass  all  along  its 
track,  and  thus  becomes  small  enough  to  again  admit  of  suspension.  It 
is  owing  to  the  influence  of  the  attraction  of  gravitation  that  the  drops  of 
different  liquids  differ  in  size,  the  heavier  yielding  small,  and  the  lighter 


ATTRACTIONS    OF   MATTER. 


large  ones.  You  have  another  illustration  of  a  similar  character  in  the 
adhesion  of  chalk  to  a  black-board  or  any  surface.  The  chalk  crayon 
itself  cannot  adhere,  for  the  attraction  of  the  earth  does  not  permit  it. 
But  small  quantities  of  it  can  adhere  for  the  same  reason  that  water  ad- 
heres to  surfaces  in  small  quantities.  Dust  also  clings  to  the  vertical  sides 
of  furniture,  though  a  lump  of  earth  would  not. 

41.  Size  of  Solid  Bodies  Limited  by  Gravitation. — We  can 
illustrate  the  limitation  of  size  in 
solid  masses  by  Figs.  34  and  35. 
Suppose  that  a  and  &,  Fig.  34,  are 
two  pieces  of  timber  projecting 
from  a  post,  b  being  twice  as 
large  as  a.  It  is  evident  that  b 
cannot  support  twice  as  much 
weight  as  «,  for  gravitation  is 
dragging  it  downward  from  its 
connection  with  the  upright  post 
with  twice  the  force  that  it  does 
a.  The  case  is  still 


Fig.  34. 


when,  as  represented  in  Fig.  35, 
the  larger  timber  is  twice  as  long 
as  the  smaller.  Here  d  has  four 
A  A  times  the  bulk  of  c.  But  it  can- 
not support  four  times  as  much 
weight  at  its  end,  not  only  be- 
cause its  own  weight  presses  it 
downward,  but  because  half  of 
its  weight  is  at  a  greater  distance 

from  the  place  of  attachment  than  the  smaller  beam  is. 
Gravitation  here  operates  in  opposition  to  cohesion  in  such 
a  way  that  the  projecting  timber,  if  carried  to  a  certain 
size,  will  fall  by  its  own  weight,  either  breaking  in  two  or 
tearing  away  from  its  attachment.  This  tendency  is  very 
commonly  resisted  in  buildings  and  other  structures  by 


76  NATUKAL   PHILOSOPHY. 

braces,  as  represented  in  Fig.  36.  Here 
the  weight  of  the  horizontal  timber  at 
some  distance  on  each  side  of  a  is  made 
to  press  upon  the  upright  post  instead 
of  directly  downward. 

The  above  Principles  Transgressed  by  Man. — 
Man  often  transgresses  these  principles  in  his  struct- 
ures. For  example,  a  building  settles  because  the 
foundation  is  not  strong  enough  to  bear  the  super- 
incumbent weight.  In  other  words,  the  force  of  gravitation  is  not  suffi- 
ciently taken  into  account.  When  a  very  tall  building  is  erected,  the 
lower  portions  ought  to  be  made  of  very  cohesive  substances.  Firm 
granite  is  therefore  an  appropriate  material  for  the  lower  story  of  tall 
brick  buildings.  At  least  the  walls  of  the  lower  stories  of  such  buildings 
should  be  made  thicker  than  usual,  to  resist  properly  the  force  of  gravita- 
tion in  the  weight  above.  Stores  intended  to  bear  much  weight  on  their 
floors  are  often  built  without  due  regard  to  the  cohesive  force  required  to 
sustain  the  weight.  Long  timbers  are  sometimes  supported  only  at  the 
ends,  when  their  own  weight,  to  say  nothing  of  what  may  be  brought  to 
press  upon  them,  requires  that  they  should  be  supported  at  other  points. 
While  in  modern  buildings  the  timbers  are  often  too  small,  in  some  old 
buildings  the  upper  timbers  are  so  heavy  as  to  lessen  rather  than  increase 
the  strength  of  the  structure.  Especially  is  this  true  of  the  unsightly 
beams  which  in  some  very  old  houses  we  see  extending  along  the  ceilings. 
Many  other  examples  could  be  given,  but  these  will  suffice. 

42.  Adhesion,  Cohesion,  and  Gravitation  the  Same. — We 
again  refer  to  the  statement  made  in  §  24,  that  cohesion, 
adhesion,  and  gravitation  are  only  different  modes  of 
action  of  the  same  power,  viz.,  the  attraction  which  mat- 
ter everywhere  has  for  matter.  At  first  thought  it  would 
appear  that  there  is  something  peculiar  in  the  attraction 
of  particles  when  they  are  brought  together  so  as  to  ad- 
here. For  if  we  take  any  substance — a  piece  of  glass,  for 
example  —  its  particles  seem  to  be  held  together  by  an 
attraction  vastly  stronger  than  that  attraction  which  in- 
clines different  bodies  to  move  towards  each  other.  If 


ATTRACTIONS    OP   MATTER.  77 

you  break  the  glass,  however  closely  you  may  press  the 
two  pieces  together,  they  will  not  unite  again.  It  would 
seem,  at  first  view,  that  there  must  be  some  peculiar  ar- 
rangement of  the  particles  which  is  destroyed  by  breaking 
the  glass.  But  we  can  readily  account  for  the  facts  in 
another  way.  The  attraction  between  bodies  of  matter 
is  greater  the  nearer  we  bring  them  together.  The  nearer, 
for  example,  the  moon  is  to  any  portion  of  the  earth,  the 
greater  the  attraction  which  it  exerts,  as  seen  in  the  tides ; 
and  if  it  were  much  nearer  to  the  earth  than  it  is,  our  tides 
would  prove  very  destructive.  What  is  true  of  masses 
is  also  true  of  the  particles  of  which  they  are  composed. 
Though  their  attraction  is  comparatively  feeble  when  at 
a  distance  from  each  other,  it  increases  not  in  the  arith- 
metical, but  the  geometrical  ratio  as  they  approach;  so 
that  when  they  are  exceedingly  near  together  the  attrac- 
tion is  very  powerful.  It  must  be  remembered  in  regard 
to  the  pieces  of  broken  glass  that  you  cannot  bring  the 
particles  on  their  surfaces  as  near  as  they  were  before  the 
glass  was  broken ;  and  the  attraction  being  inversely  as 
the  square  of  the  distance,  a  little  distance  must  make  a 
great  difference.  The  particles  of  some  substances  you 
can  bring  so  near  together  as  to  cause  adhesion,  as  in  the 
case  of  the  two  bullets  (§  30).  That  their  adhesion  depends 
merely  upon  their  particles  being  brought  near  to  each 
other  appears  from  the  fact  that  the  smoother  you  make 
the  surfaces,  the  more  strongly  will  they  adhere.  And  the 
reason  that  liquids  and  semi-liquids  adhere  so  readily  to 
solid  substances  is  that  their  particles,  moving  freely  among 
each  other,  have  thus  the  power  of  arranging  themselves 
very  near  to  the  particles  of  the  solid.  Thus,  when  a  drop 
of  water  hangs  to  glass,  all  the  particles  of  water  in  that 
part  of  the  drop  next  to  the  glass  touch,  or  rather  are 
exceedingly  near  to,  the  particles  of  the  glass. 

D 


78  NATURAL   PHILOSOPHY. 

43.  Chemical  Attraction. — The  kinds  of  attraction  hith- 
erto explained  in  this  work  belong  to  the  study  of  Natu- 
ral Philosophy,  but  another  kind,  known  as  interatomic  or 
chemical  attraction,  is  capable  of  producing  the  most  won- 
derful effects.  The  former  produce  chiefly  mechanical  ef- 
fects, while  chemical  attraction  goes  farther  and  affects  the 
composition  of  substances.  Eor  example,  the  attraction  be- 
tween the  two  gases  oxygen  and  hydrogen,  which  makes 
them  combine  to  form  water,  belongs  to  Chemistry;  while 
that  which  makes  the  particles  of  water  cohere  is  in  the 
province  of  Natural  Philosophy.  You  will  learn  more 
about  chemical  attraction  in  Part  II.  of  this  series. 

Variety  in  the  Results  of  Attraction. — It  is  one  and  the 
same  force,  then,  which  binds  the  particles  of  a  pebble 
together,  and  makes  it  fall  to  the  ground — which  "  moulds 
the  tear"  and  "bids  it  trickle  from  its  source"  —  which 
gives  the  earth  and  all  the  heavenly  bodies  their  globular 
shape,  and  makes  them  revolve  in  their  orbits.  How 
sublime  the  thought  that  one  simple  principle  which  gives 
form  to  a  drop  extends  its  influence  through  the  immensity 
of  space,  and  so  marshals  "the  host  of  heaven"  that,  with- 
out the  least  interruption  or  discord,  they  all  hold  on  their 
course  from  year  to  year  and  from  age  to  age !  Thus  Om- 
nipotence makes  the  simplest  means  produce  the  grandest 
and  most  multiform  results. 

/ 

QUESTIONS. 

36.  What  is  adhesion  ?  Why  can  you  not  make  the  surfaces  of  broken 
glass  adhere?  Explain  the  cementing  of  glass.  How  may  silver  and  gold 
be  made  to  adhere  to  iron  ?  What  is  said  of  the  adhesion  of  tin  and  lead  ? 
— 37.  What  is  said  of  the  adhesion  of  liquids  to  solids  ?  What  of  the  ac- 
tion of  mercury  ?  Describe  an  experiment  showing  the  adhesion  of  solids 
and  liquids.  What  is  said  of  stems  in  stagnant  water?  Explain  Figs.  23, 
24,  and  25.  Explain  Fig.  27. — 38.  Explain  the  rise  of  fluids  in  tubes  by 


CENTRE    OP   GRAVITY. 


79 


Fig.  28.  How  does  mercury  act  with  respect  to  tubes  plunged  into  it  ? — 
39.  What  is  meant  by  capillary  attraction?  Give  familiar  examples  of 
the  rising  of  liquids  in  interstices.  Describe  and  explain  the  process  of 
getting  out  millstones.  How  does  a  blotter  differ  from  writing-paper? 
Describe  the  experiment  with  a  sewing-needle.  What  is  said  of  certain 
water  insects  ?— 40.  What  is  said  of  the  various  results  of  attraction?  Ex- 
plain fully  why  you  can  pour  water  from  a  pitcher  easier  than  from  a  tum- 
bler. Explain  the  operation  of  the  quick  movement  by  which  you  prevent 
water  from  running  down  the  side  of  a  tumbler  in  pouring  it  out.  What  is 
said  of  dropping  from  a  vial  ?  How  is  the  size  of  drops  limited  ?  What 
is  said  of  the  movements  of  drops  on  window-panes  ?  Why  do  the  drops 
of  different  liquids  vary  in  size?  Give  the  illustration  about  chalk. 
Give  that  about  dust. — 41.  Illustrate  the  limitation  of  size  in  solid  masses. 
Show  how  these  principles  are  transgressed  by  man. — 42.  Give  the  sum- 
mary referring  to  the  connection  between  the  different  ways  in  which  at- 
traction is  exerted. — 43.  Wherein  does  chemical  attraction  differ  from  the 
other  kinds  ?  What  is  said  of  the  variety  in  the  results  of  attraction? 


CHAPTER  VI. 

CENTRE    OF    GRAVITY. 

44.  Centre  of  Gravity  Illustrated. — If  you  support  a  ruler 

on  your  finger  as  in  Fig.  37, 

it  balances  when  there  is  just 
as  much  weight  on  one  side  as 
on  the  other.  Now  just  over 
your  finger,  in  the  middle  of 
the  ruler,  there  is  a  point  called 
the  centre  of  gravity;  or,  in 
other  words,  the  centre  of  the 
weight  of  the  ruler.  This  point 
is  indicated  in  the  figure. 
There  is  as  much  of  the  weight 
of  the  ruler  on  the  one  side  of 


80 


NATURAL   PHILOSOPHY. 


this  point  as  on  the  other,  and  also  as  much  above  it  as  be- 
low it.  If  your  finger  should  be  a  little  to  the  one  side  or 
the  other  of  this  point,  the  ruler  would  not  be  balanced,  and 
would  fall.  When  balanced,  it  does  not  fall,  simply  because 
this  central  point  is  supported  by  being  directly  over  the 
end  of  the  finger.  The  whole  weight  of  the  ruler,  then, 
may  be  considered  as  practically  concentrated  at  that  point, 
for  all  the  downward  pressure  of  the  ruler  is  there  exert- 
ed. When  the  ruler  is  balanced  on  the  finger  as  repre- 
sented in  Fig.  38,  it  will  maintain 
its  position  so  long  as  its  centre  of 
gravity  is  directly  over  the  point 
of  the  finger.  If  it  be  to  the  one 
side  or  the  other,  as  in  Fig.  39, 
it  is  not  supported,  and  the  ruler 
therefore  falls.  You  see,  then,  that 
when  a  body  is  balanced,  the  cen- 
tre of  gravity  lies  directly  over  the 
point  of  support.  If,  on  the  other 
hand,  a  body  is  suspended,  the  cen- 
tre of  gravity  is  directly  under  the 
point  of  support. 

If  a  plumb-line  from  the  centre 
of  gravity  of  any  body  could  be 
prolonged  into  the  earth,  it  would 
go  directly  to  its  centre.  The  body 
may  be  considered  as  making  all  its 
pressure  from  its  centre  of  gravity 
towards  the  centre  of  the  earth,  in 
obedience  to  the  attraction  of  gravitation.  The  best 
definition,  then,  that  we  can  give  of  the  centre  of  grav- 
ity is,  that  point  in  a  body  from  which  proceeds  its 
pressure  as  a  whole  toicards  the  centre  of  the  earth.  It 
is  that  point,  therefore,  the  support  of  which  insures  the 


Fig.  33. 


Fig.  39. 


CENTRE    OF   GRAVITY. 


81 


Fig.  40. 


support  of  the  whole  body.  And  in  speaking  of  the 
weight  of  a  body,  or  its  downward  pressure,  we  may  con- 
sider all  the  matter  composing  it  as  collected  or  concen- 
trated in  that  point.  The  body,  therefore,  can  be  balanced 
in  any  position  in  which  this  point  is  supported,  as  shown 
in  Figs.  37  and  38.  When  a  body  is  suspended,  it  is  at 
rest  only  when  the  centre  of  gravity  is  directly  under  the 
point  of  support.  Thus,  if  you  have  a  circu- 
lar plate  suspended  at  E,  Fig.  40,  it  will  not 
be  at  rest  when  moved  to  the  one  side  or  the 
other,  as  represented  by  the  dotted  lines,  but 
only  when  the  centre  of  gravity,  c,  is  directly 
under  the  point  E. 

45.  How  to  Find  the  Centre  of  Gravity  of  a  Body.— If  you 
take  a  piece  of  board,  and  suspend  it  at  a,  Fig.  41,  and 

hang  a  plumb-line 
from  the  same  point, 
the  centre  must  be 
somewhere  in  that 
line.  But  exactly  at 
what  point  it  is  you 
do  not  know.  How 
will  you  ascertain 
this?  Mark  the  line 
a  c  on  the  board,  and 

suspend  the  board  by  another  point,  as  in 
Fig. 41.  Yig.  42.  Since  the  centre  of  gravity  must 
be  somewhere  in  the  plumb-line  as  it  now  hangs,  of  course 
it  is  where  the  two  lines  ac  and  bd  cross,  and  the  board 
suspended  by  a  cord  attached  at  this  point  will  remain 
evenly  balanced. 

Scales  and  Steelyards. — When  two  bodies  are  connected  by  a  rod  or 
bar,  the  centre  of  gravity  of  the  whole  is  somewhere  in  the  connecting  rod. 
If  the  two  bodies  be  equal  in  weight,  as  in  Fig.  43,  the  centre  of  gravity  is 


82  NATURAL   PHILOSOPHY. 

exactly  in  the  middle  of  the  rod,  as  marked.     But  if  the  bodies  are  un- 
equal, as  in  Fig.  44,  the  centre  of  gravity  is  nearer  to  the  larger  body  than 


e 


Fig.  43.  Fig.  44. 

to  the  smaller.  In  weighing  a  body  in  one  pan  of  a  balance  by  means  of 
weights  placed  in  the  other,  we  have  a  case  parallel  to  that  of  Fig.  43.  The 
centre  of  gravity  of  the  body  weighed,  the  weights  and  the  pans,  as  a  whole, 
is  midway  between  the  scales,  at  the  point  of  support.  In  the  steelyard 
the  heavy  body  to  be  weighed  is  nearer  the  centre  of  gravity  than  the  small 
weight  on  the  long  arm,  and  so  the  case  is  similar  to  that  of  Fig.  44. 

46.  The  Centre  of  Gravity  of  a  Body  not  Always  in  the 
Body  itself. — The  centre  of  gravity  of  a  hollow  ball  of 
uniform  thickness  is  not  in  the  substance  of  the  ball,  but 
it  is  in  the  centre  of  the  space  within  the  ball,  for  the  line 
of  the  ball's  downward  pressure  is  situated  at  that  point. 
If  the  ball  had  a  framework  in  it,  as  represented  in  Fig.  45, 

the  centre  of  gravity  would  obvious- 
ly be  at  A,  the  centre  of  this  frame- 
work. But  if  there  were  no  frame- 
work, and  perpendicular  lines  were 
supposed  to  be  drawn  from  different 
points  of  suspension,  C,  B,  D,  and  E, 
these  wTould  intersect  at  the  point  A, 
showing  that  this  is  the  centre  of 
gravity,  according  to  the  rule  for 

finding  it  given  in  §  45.  In  like  manner,  the  centre  of 
gravity  of  an  empty  box,  or  of  an  empty  ship,  is  an  imag- 
inary point  in  the  space  inside.  In  a  hoop  it  is  the  centre 
of  the  hoop's  circle. 

47.  The  Centre  of  Gravity  Seeks  the  Lowest  Point.  — 
The  centre  of  gravity  always  assumes  the  lowest  place 
which  the  support  of  the  body  will  allow.     In  a  hanging 
body,  therefore,  it  is  always  directly  under  the  point  of 
suspension.    To  reach  one  side  or  the  other  of  this  posi- 


CENTRE    OF   GRAVITY.  83 

tion,  it  must  rise.  This  the  attraction  of 
gravitation  forbids,  and  if  by  any  force  it  is 
made  to  rise,  this  attraction  at  once  brings  / 

it  back.     This  is  manifest  in  the  case  of  a  / 

suspended   ball,   Fig.  46.     If  the  ball  be 
moved  to  b,  it  will,  on  being  let  go,  return  (    ^ 
to    its   first   position,   simply   because   its   -," 
centre  of  gravity,  in  obedience  to  the  earth's  ^^ 

attraction,  seeks  the  lowest  place,  possible. 
From  inertia  (§  14),  it  moves  beyond  this  point,  and  con- 
tinues to  vibrate  back  and  forth  for  some  time ;  but  when 
its  motion  is  stopped,  it  hangs  perpendicularly ;  that  is,  in 
such  a  way  that  its  centre  of  gravity  shall  have  the  lowest 
possible  position.  Many  illustrations  of  this  point  might 
be  mentioned. 

When  a  rocking-horse  is  at  rest,  its  centre  of  gravity  is  directly  over  the 
point  at  which  it  touches  the  floor,  for  in  that  position  the  centre  of  grav- 
ity is  as  low  as  possible.  If  it  be  rocked,  the  centre  of  gravity  is  moved  to 
a  higher  point,  and  for  this  reason  it  rocks  back  again.  The  same  is  seen 
in  the  swing,  the  cradle,  the  rocking-chair,  etc.  Most  interesting  illustra- 
tions are  found  in  the  Loggan  Stones,  as  they  are  called,  several  of  which 
are  seen  on  the  rugged  parts  of  the  British  coast.  An  immense  rock, 
loosened  by  some  of  the  forces  of  nature,  rests  with  a  slightly  rounded 
base  on  another  rock  which  is  flat,  and  it  is  so  nicely  balanced  that  one 
person  alone  has  sufficient  strength  to  set  it  rocking.  Similarly  balanced 
"rocking-stones"  are  found  near  Salem,  Massachusetts,  in  Great  Barring- 
ton,  Massachusetts,  and  in  many  other  localities. 

48.  Further  Illustrations. — An  egg  lies  upon  its  side  be- 
cause the  centre  of  gravity  seeks  the  lowest  position. 
When  on  its  side,  the  centre  of  gravity  is  at  its  lowest 
point,  as  is  manifest  by  a  comparison  of  Fig.  47  with  Fig. 
48.  Children  have  a  toy,  called  a  witch,  which  illustrates 
the  same  thing  in  another  way.  It  is  constructed  of  some 
light  substance,  as  pith,  with  a  shot  or  bullet  fastened  in 
one  end.  It  always  stands  up  on  its  loaded  end,  and  can- 


84 


NATURAL   PHILOSOPHY. 


Fig.  47. 


Fiff.  48. 


not  be  made  to  lie  down  on  its  side,  because  the  centre 
of  gravity  would  not  then  b6  at  the  lowest  point.  The 
figure  of  a  fat  old  woman,  Fig.  49,  loaded  with  lead  at 
the  bottom,  is  another  form  of  this  toy.  If  the  figure  be 
thrust  over  to  one  side,  as  shown  by  the  dotted  lines, 

the  centre  of 
gravity  is 
raised,  and 
the  upright 
position  is  at 


.  so. 


once  resumed. 


Fig.  49. 


If  the  toy  were  not  loaded,  it  would 

lie  in  the  position  represented  in  Fig. 

50,  just  as  the  egg  lies  on  its  side. 
Certain  curious  cases  which  at  first  sight  appear  to  be 
exceptions  to  this  law  are  really  interesting  proofs  of  it. 
If  a  light  wooden  cylinder  loaded  with  lead  on  one  side 
be  placed  upon  an  inclined  plane  with  the  lead  in  the  po- 
sition e  (Fig.  51),  the  lead,  in  fall- 
ing to  o,  will  cause  the  cylinder  to 

roll  up  the  incline.    What  is  ap-  \        ^/ .& 

parently  a  rolling  up-hill  is  really 

a  falling  of  the  centre  of  gravity.  Fig.  51. 

For  the  same  reason  a  billiard-ball  placed  on  the  smaller 
ends  of  two  billiard-cues  laid  on  a  table  with  their  points 


CENTRE    OF    GRAVITY. 


85 


in  contact  and  their  larger 
ends  slightly  separated,  will 
roll  towards  the  large  ends, 
apparently  rolling  up-hill; 
actually,  however,  the  cen- 
tre of  gravity  falls  as  the 
ball  rolls  alonec  the  cues. 


Fig.  52. 
But  if  you  place   another   stick   as   a  brace,  in 


Curious  Experiments. — You  can- 
not hang  a  pail  of  water  on  a  stick 
laid  upon  a  table  as  represented 
in  Fig.  52,  for  the  centre  of  grav- 
ity is  not  supported. 
the  manner  represented  in  Fig.  53,  so  as  to  push  the  pail  under  the  table, 

it  will  hang  securely,  because  the 
centre  of  gravity  will  be  brought 
under  the  point  of  suspension.  In 
an  entirely  similar 
manner  a  needle 
passed  through  a 
cork  into  which  a 
fork  is  thrust  (as 
shown  in  Fig.  54), 
may  be  suspended 
on  the  edge  of  a 
table.  We  have 


Fig.  53. 


another    illustra- 


tion in  the  common  toy  represented  in  Fig. 
55.  The  horse,  made  of  very  light  material, 
stands  securely,  because  the  centre  of  grav- 
ity of  the  whole  is  in  the  heavy  ball,  which  is 
under  the  point  of  suspension.  If  the  horse 
be  made  to  rock  back  and  forth,  the  centre  of 
gravity  in  the  ball  moves  in  a  curved  line,  as 
in  the  case  of  a  ball  suspended  by  a  string  (Fig. 
46).  It  is  at  its  lowest  place  only  when  the 
horse  is  at  rest.  The  hanging  of  a  cane  with 
a  hook-shaped  handle  on  the  edge  of  a  table 
is  to  be  explained  in  the  same  way. 

D2 


Fig.  54. 


Fig.  55. 


86 


NATURAL   PHILOSOPHY. 


-  49.  Stability  of  Bodies. — The  firmness  with  which  a  body 
stands  depends  upon  two  circumstances — the  height  of  its 
centre  of  gravity  and  the  extent  of  its  base.  The  lower  the 
centre  of  gravity,  and  the  broader  the  base,  the  firmer  the 
body  stands.  A  cube,  represented  in  Fig.  56,  is  more  stable 
—that  is,  less  easily  turned  over — than  a  body  shaped  like 
that  in  Fig.  57,  because  it  has  a  larger  base.  The  contrast 
is  still  greater  between  Figs.  56  and  58.  The  reason  of  the 
stability  of  a  body  with  a  broad  base  is  found  in  the  fact 
that  in  turning  it  over  the  centre  of  gravity  must  be 
raised  more  than  in  turning  over  one  of  a  narrower  base. 
The  curved  lines  indicate  the  paths  of  the  centres  of  grav- 


Fig.  56. 


Fig.  57. 


Fig.  58. 


ity  as  the  bodies  are  turned  over.  In  the  case  of  a  per- 
fectly round  ball,  the  base  is  a  mere  point,  and  therefore 
the  least  touch  turns  it  over.  Its  centre  of  gravity  does 
not  rise  at  all,  but  moves  in  a  horizontal  line,  as  shown  in 
Fig.  59.  The  pyramid  is  the  firmest  possible  structure,  be- 


CENTRE    OF   GRAVITY. 


87 


cause  it  possesses  in  the  highest  degree  the  two  elements 
a  broad  base  and  a  low  position  of  the  centre  of  gravity. 
On  both  these  accounts  the  centre  of  gravity  must  ascend 
considerably  when  the  body  is  turned  over,  as  shown  in 
Fig.  60. 

50.  Unstable  Bodies. — When  a  body 
does  not  stand  upright,  its  stability  is 
diminished  because  only  a  portion  of 
the  base  is  concerned  in  its  support. 
In  Fig.  61  the  base  is  broad,  but  the 
body  is  so  far  from  being  upright  that  / 
the  centre  of  gravity  bears  upon  the  - 
very  extremity  of  the  base  on  one 
side,  as  indicated  by  the  perpendicular  line.  A  small  force 
will  turn  it  over,  because  the  centre  of  gravity  need  not  as- 
cend the  least  when  this  is  done.  You  see,  then,  that  the 
less  upright  a  body  is,  the  less  of  the  base  is  of  service  in 

its  support.  The  famous 
tower  of  Pisa,  Fig.  62, 
one  hundred  and  thirty 
feet  high,  overhangs  its 
base  fifteen  feet.  Wheth- 
er it  was  built  intention- 
ally in  this  way  to  excite 
wonder  and  surprise,  or 
whether  it  settled  on  one 
side  after  its  completion, 
has  long  been  discussed ; 
we  are  inclined  to  the 
former  theory,  for  what 
would  otherwise  have 
been  a  very  unsafe 
structure  is  rendered  sta- 
ble and  safe  by  the  arrangement  of  its  materials.  Its 


NATURAL   PHILOSOPHY. 

lower  portion  is  built  of  very  dense  rock,  the  middle  of 
brick,  and  the  upper  of  a  very  light  porous  stone.  In  this 
way  the  centre  of  gravity  of  the  whole  structure  is  made 
to  have  a  very  low  position. 

Familiar  Illustrations.  — You  are  now  prepared  to  understand  a  fact 
which  common  experience  teaches  every  one,  that  the  taller  a  body,  and  the 

narrower  its  base,  the  more  easily  is  it 
overturned.  This  is  exemplified  in  the 
twq  loads,  Fig.  63.  The  base  is  the 
space  included  by  the  wheels.  The 
centre  of  gravity  is  so  high  in  the  tall 
load  that  a  perpendicular  line  drawn 
from  it  falls  outside  of  the  base  if  the 
cart  reaches  a  considerable  lateral  in- 
.  C3.  clrnation  of  the  road.  But  the  small- 

er load,  under  the  same  circumstances,  is  perfectly  secure.  A  high  car- 
riage is  more  easily  overturned  than  a  low  one,  for  the  same  reason. 
A  stage,  if  overloaded  on  its  top,  is  very  unsafe  on  a  rough  road.  Sta- 
bility is  given  to  articles  of  furniture  by  making  their  bases  broad  and 
heavy,  as  shown  in  tables  supported  by  a  central  pillar,  candlesticks, 
lamps,  etc.  The  tall  chairs  in  which  children  sit  at  table  would  be  very 
insecure  if  the  legs  were  not  widely  separated  at  the  bottom,  thus  widening 
the  base  of  support. 

51.  Support  of  the  Centre  of  Gravity  in  Animals. — The 
base  of  support  which  quadrupeds  have,  viz.,  the  space  in- 
cluded between  their  four  feet,  is  quite  large ;  and  this  is 
one  reason  why  they  are  able  to  walk  while  yet  very  young. 
A  child  does  well  who  can  walk  at  the  end  often  or  twelve 
months,  for  the  supporting  base  is  quite  small  compared 
with  that  of  a  quadruped.  It  requires  skill,  therefore,  in  the 
child  to  manage  the  centre  of  gravity  in  standing  and  walk- 
ing, and  this  is  gradually  acquired.  It  is  on  account  of  the 
smallness  of  the  base  furnished  by  the  feet  that  the  statue 
of  a  man  is  always  made  with  a  large  base  or  pedestal. 
Although  we  exert  considerable  skill  in  walking,  it  is  by 
no  means  so  great  as  that  which  the  Chinese  ladies  require 


CENTEE    OF    GRAVITY. 


89 


with  their  painfully  small  feet.  Still  more  skill  is  exercised 
by  one  who  has  two  wooden,  legs,  or  one  who  walks  on 
stilts.  The  base  made  by  the  feet  can  be  varied  much  by 
their  position.  If  the  toes  be  turned  out  and  the  heels 
brought  near  to  each  other,  the  base 
will  not  be  so  large  as  when  the  feet 
are  straight  forward  and  far  apart,  as 
is  manifest  in  Fi^s.  64  and  65.  It  is  for 


Fig.  64. 


05. 


this  reason  that  the  child,  in  his  first  attempts  at  standing 
and  walking,  instinctively  manages  his  feet  as  in  Fig.  64. 

52.  Motions  of  the  Centre  of  Gravity  in  Walking.  —  In 
walking,  the  centre  of  gravity  is  alternately  brought  over 
one  foot  and  the  other,  and  so  moves  in  a  waving  line.  This 
is  very  manifest  as  you  see  people  before  you  going  down 
the  aisle  out  of  a  church.  When  two  are  walking  together, 
if  they  keep  step  the  two  waving  lines  of  their  centres  of 
gravity  run  parallel,  as  in  Fig.  66,  and  they  walk  easily ; 


Fig.  60. 

but  if  they  do  not  keep  step  these  lines  run  as  in  Fig.  6V, 
and  the   movement  is  both  awkward  and  embarrassing. 


This  line  of  movement  of  the  centre  of  gravity  is  always 
slightly  waving  upward  also,  as  seen  in  Fig.  68.     In  the 


Fig.  63. 


NATURAL  PHILOSOPHY. 

case  of  a  man  with  wooden  legs,  the  line  would  not  be 
gently  waving,  but  somewhat  angular,  as  represented  in 
Fig.  69. 

53.  The  Centre  of  Gravity  and  Attitudes. — The  object  of 
various  attitudes  assumed  under  different  circumstances  is 
to  keep  the  centre  of  gravity  over  the  base  of  support.  A 
man  with  a  load  on  his  back  would  not  stand  straight,  but 
would  assume  the  position  of  Fig.  70,  so  that  the  centre 
of  gravity  of  his  load  may  be  directly  over  his  feet.  A 
man  carrying  a  pail  of  water  in  his  left  hand  leans  to  the 
right,  and  raises  his  right  hand  in  order  to  bring  the  centre 
of  gravity  over  his  feet  (Fig.  71).  In  ascending  a  hill  a 
man  appears  to  lean  forward,  and  in  descending  to  lean 


Fiir.  70. 


Fig.  Tl. 


backward ;  but  in  fact  he  is  in  both  cases  upright  in  refer- 
ence to  the  plain  on  which  the  hill  stands.  A  perpendicu- 
lar line  drawn  from  his  centre  of  gravity  strikes  the  ground 
midway  between  his  feet,  that  is,  in  the  middle  of  the  base, 
and  if  prolonged  would  go  straight  to  the  centre  of  the 
earth.  When  one  rises  from  a  chair  he  draws  his  feet  back- 
ward, and  then  bends  his  body  forward  to  bring  the  centre 


CENTRE    OF   GEAVITY.  91 

of  gravity  over  the  feet.  Unless  this  is  done,  it  is  impos- 
sible to  rise,  at  least  deliberately,  as  you  will  find  by  trying 
the  experiment.  A  man  standing  with  his  heels  close  to  a 
wall  cannot  stoop  forward  and  pick  up  anything,  for  the 
Avail  prevents  him  from  moving  any  part  of  his  body  back- 
ward, and  therefore  when  he  stoops  forward,  the  centre  of 
gravity  being  brought  in  advance  of  the  base,  he  loses  his 
balance  and  falls.  A  man  who  did  not  understand  this 
undertook  to  stoop  in  this  way  to  pick  up  a  purse  contain- 
ing twenty  guineas,  which  he  was  to  win  if  he  succeeded, 
the  forfeiture  in  case  of  failure  being  ten  guineas.  Of  course 
his  lack  of  knowledge  as  to  the  principles  of  the  centre  of 
gravity  made  him  lose  his  wager. 

Great  skill  is  exhibited  by  the  rope-dancer  in  supporting  the  centre  of 
gravity.  He  carries  a  long  pole  in  his  hands,  loaded  at  each  end,  and 
when  he  inclines  to  one  side  he  throws  it  a  little  towards  the  other  side,  that 
the  reaction  may  restore  his  balance.  Similar  skill  is  seen  in  feats  of  bal- 
ancing, as,  for  example,  in  balancing  a  long  stick  upright  on  the  finger. 
In  these  cases  the  centre  of  gravity  is  very  little  of  the  time  directly  over 
the  point  of  support.  It  is  kept  in  constant  motion  nearly  but  not  quite 
over  this  point — this  unstable  equilibrium,  as  it  is  called,  being  vastly  less 
difficult  to  maintain  than  stable  equilibrium ;  that  is,  keeping  the  balance 
in  one  unvarying  position.  It  is  the  motion  of  the  top  that  makes  it  stand 
upright  upon  its  point — a  very  beautiful  example  of  unstable  equilibrium. 
The  centre  of  gravity  revolves  around  a  perpendicular  line,  at  exceedingly 
little  distance  from  it  at  first,  but  greater  and  greater  as  its  motion  be- 
comes less  rapid,  till  at  length  the  centre  of  gravity  gets  so  far  from  this 
line  that  the  top  falls.  For  a  similar  reason  an  intoxicated  man  may  not 
be  able  to  keep  himself  up  if  he  undertakes  to  sjand  still,  and  yet  may  do 
so  if  he  keep  moving. 

54.  Centre  of  Gravity  in  Floating  Bodies. — The  same  prin- 
ciples wrhich  apply  to  the  centre  of  gravity  in  bodies  stand- 
ing on  a  firm  basis  apply  also  to  floating  bodies.  That  the 
centre  of  gravity  may  be  low  in  a  loaded  vessel  the  heavy 
part  of  the  cargo  is  put  underneath,  and  generally  ballast 
of  stone  or  iron  is  necessary  for  the  same  purpose.  In  large 


92  NATURAL  PHILOSOPHY. 

flat-boats,  the  base  of  support  being  extensive,  there  is  not 
the  same  need  of  taking  care  that  the  centre  of  gravity  be 
low.  If  a  ship  be  laden  in  part  with  an  article  which  will 
dissolve  in  water,  there  is  much  danger,  if  the  ship  should 
leak,  lest  this  portion  of  the  cargo  be  dissolved  and  pumped 
out  with  the  bilge -water;  this  would  alter  the  trim  of 
the  vessel  by  removing  the  centre  of  gravity  from  over 
the  middle  line,  and  bringing  it  too  far  forward  or  carry- 
ing it  too  far  back,  making  the  ship  wholly  unmanageable. 
Four  large  English  ships,  in  part  loaded  with  saltpetre,  were 
supposed  to  have  been  lost  from  this  cause  in  1809  oif  the 
Isle  of  France.  The  immense  ice-islands,  or  icebergs,  which 
float  about  in  summer  in  the  polar  regions,  by  melting  ir- 
regularly often  change  the  place  of  their  centre  of  gravity, 
and  in  turning  over  present  one  of  the  most  sublime  spec- 
tacles in  nature.  A  mountain  of  ice,  extending  high  in  the 
air  and  deep  in  the  sea,  suddenly  turns  over,  and  produces 
a  rolling  of  the  ocean  which  is  often  felt  at  the  distance  of 
many  leagues. 

QUESTIONS. 

44.  Show  what  we  mean  by  the  centre  of  gravity  by  Figs.  37,  38,  and 
39.  Give  the  definition  of  centre  of  gravity,  and  explain  it.  What  is 
shown  by  Fig.  40  ? — 45.  How  can  we  find  the  centre  of  gravity  of  a  body  ? 
What  is  said  of  scales  and  steelyards  ? — 4G.  State  what  is  represented  by 
Fig.  45. — 47.  Illustrate  the  fact  that  the  centre  of  gravity  seeks  always  the 
lowest  point.  Give  the  illustrations  of  the  rocking-horse,  the  swing,  etc. 
What  is  said  of  the  Laggan  Stones  ? — 48.  Why  does  an  egg  lie  on  its 
side?  Give  the  illustrations  from  toys.  Explain  how  a  ball  may  be  made 
to  roll  up  an  incline.  Describe  the  experiment  with  a  pail.  And  that 
with  a  toy  horse. — 49.  Upon  what  two  things  does  the  stability  of  a  body 
depend  ?  What  is  said  of  the  stability  of  bodies  whose  shapes  are  repre- 
sented in  Figs.  56,  57,  and  58  ?  WThat  of  that  of  a  round  ball  ?  Why  is  the 
pyramid  the  firmest  of  all  structures  ? — 50.  What  is  the  relation  of  upright 
position  to  stability  ?  What  is  stated  of  the  tower  of  Pisa  ?  Give  some 
familiar  illustrations. — 51.  What  is  said  of  the  support  of  the  centre  of 


MOTIONS   OF  MATTER.  93 

gravity  in  animals  ?  What  is  said  of  the  skill  exercised  in  walking  ?  What 
of  the  mode  of  walking  in  a  child  ? — 52.  What  of  the  motions  of  the  centre 
of  gravity  in  walking  ?  What  is  said  of  the  walking  of  a  man  with  wooden 
legs  ? — 53.  Illustrate  the  management  of  the  centre  of  gravity  in  different 
attitudes.  Describe  and  explain  the  way  in  which  one  rises  from  a  chair. 
State  and  explain  the  wager  case.  What  is  said  of  unstable  equilibrium  ? 
Give  the  illustrations. — 54.  What  of  the  centre  of  gravity  in  floating  bodies? 
What  is  said  of  icebergs  ? 


CHAPTER  VII. 

K 

MOTIONS     OF     MATTER. 

55.  Matter,  Motion,  Force. — When  a  ball  is  rolled  over 
the  floor  a  superficial  observer  sees  but  little  occasion  for 
scientific  discussion,  but  a  philosophical  mind  finds  therein 
a  symbolic  illustration  of  certain  phenomena  of  nature 
ever  present  with  us,  and  a  comprehension  of  which  is 
of  the  highest  importance.  Were  the  sentence,  "A  ball 
rolls,"  critically  analyzed  by  a  student  of  grammar,  he 
would  tell  us  of  the  three  parts  of  speech  represented,  and 
of  their  relation  to  each  other;  in  like  manner  the  stu- 
dent of  the  laws  of  nature,  analyzing  the  same  sentence, 
would  tell  us  that  it  embodies  three  facts,  and  that  their 
mutual  relations,  intelligently  studied,  cover  the  whole 
groundwork  of  Natural  Philosophy.  We  will  endeavor 
to  explain  our  meaning  by  dissecting  the  sentence. 

"  A  ball  rolls  "  leads  us,  in  the  first  place,  to  consider  the 
ball  itself;  the  phrase  being  indefinite,  we  have  no  informa- 
tion as  to  the  material  of  which  the  ball  is  made,  whether 
of  wood,  iron,  or  rubber;  whatever  the  substance  may  be, 
it  is  called  matter,  as  explained  in  Chapter  I.  The  ball, 
then,  abstractly  considered,  is  merely  an  indefinite  quantity 


94  NATURAL   PHILOSOPHY. 

of  matter  having  a  spherical  form,  the  latter  idea  being  as- 
sociated with  the  term  ball  itself. 

In  the  second  place,  this  sentence  leads  us  to  regard  the 
ball  as  changing  its  place  with  reference  to  some  other 
body  not  mentioned.  The  ball  "rolls,"  i.e.,  is  in  motion, 
or  moves  from  one  place  to  another  in  a  particular  manner 
known  as  rolling.  Motion,  then,  in  the  abstract,  is  a  change 
of  place,  and  is  so  defined.  Now  we  have  already  learned 
that  matter  is  of  itself  inert  (§  14),  and  cannot  put  itself  in 
motion,  hence  there  must  be  some  external  cause  for  the 
rolling  of  the  ball;  and  this  leads  us  to  the  third  point,  viz., 
the  idea  of  force.  That  which  causes  motion  in  matter  is 
called  force :  the  force  of  the  explosion  of  gunpowder  sets 
the  bullet  (matter)  in  motion;  \\\Q  force  of  a  violent  wind 
uprooting  a  tree  also  sets  matter  in  inotion.  A  clear  un- 
derstanding of  these  three  phenomena,  and  of  their  mutual 
relations  as  governed  by  laws,  is  essential  to  the  study  of 
Natural  Philosophy,  and  their  discussion  will  occupy  us 
throughout  this  Chapter. 

Force. — We  have  already  stated  that  force  is  that  which 
tends  to  move  matter.  When  a  body  begins  to  move, 
changes  the  style  of  motion  it  acquired,  or  ceases  to  move, 
it  is  the  result  of  one  or  more  forces  acting  upon  it  from 
without.  Force  is  not  an  attribute  of  matter  like  divisi- 
bility or  hardness  (Chapter  II.),  but  merely  a  tendency 
to  put  it  in  motion ;  we  say  a  tendency,  because  force  may 
exist  where  there  is  no  actual  motion.  Thus  a  huge  rock 
may  rest  quietly  for  years  on  the  sloping  hill-side,  prevent- 
ed from  moving  by  a  small  quantity  of  earth  in  front  of  it, 
but  let  this  obstacle  be  removed  by  shovelling,  or  by  a  sud- 
den flood  of  water,  and  the  rock  will  roll  down  the  hill  with 
immense  force,  crushing  everything  in  its  path.  The  mag- 
net, about  which  you  will  learn  more  in  Chapter  XX.,  af- 
fords another  illustration  of  the  correct  idea  conveyed  by  the 


MOTIONS    OF   MATTEK.  95 

word  force.  A  magnet  has  the  power  of  attracting  to  itself 
pieces  of  iron ;  if  the  magnet  lies  on  the  table,  and  no  iron 
objects  are  near  it,  the  fact  of  its  possessing  this  peculiar 
force  is  not  apparent — the  force  is  sleeping  as  we  may 
say;  but  bring  near  to  the  magnet  some  iron  nails  or 
some  steel  filings,  and  this  sleeping  force  is  aroused,  and 
manifests  itself  by  drawing  the  iron  articles  towards  the 
magnet. 

56.  Motion  Universal. — The  material  universe  is  in  cease- 
less motion.  The  rising  and  setting  of  the  sun,  the  changes 
of  the  seasons,  the  falling  of  the  rain,  the  running  of 
rivers  into  the  ocean,  the  ascent  of  water  into  the  air  by 
evaporation,  the  wind  moving  in  silence  or  rushing  on  in 
its  might,  are  familiar  examples  of  motion  constant  and 
everywhere  present.  But  with  all  this  motion,  sometimes 
in  conflict  and  often  variable,  order  and  regularity  reign. 
The  forces  causing  motion,  though  various  in  their  opera- 
tion, are  kept  by  the  Creator  from  producing  confusion  and 
disorganization  by  a  few  simple  laws,  which  regulate  the 
movements  both  of  atoms  and  of  worlds. 

The  principal  of  these  causes  of  motion  are  the  forces  mentioned  in 
Chapter  IV.  ;  we  will  briefly  recapitulate  them.  Attraction  is  the  most 
universal  of  the  causes  of  motion  in  the  universe.  While  it  binds  atom  to 
atom,  it  also  binds  system  to  system  throughout  the  immensity  of  space ; 
and  while  it  makes  the  stone  fall  to  the  ground,  it  moves  the  countless 
orbs  forever  onward  in  their  courses.  It  is  this  which  causes  the  tides  to 
flow  and  the  rivers  to  run  down  their  slopes  to  the  ocean,  and  thus  by 
keeping  up  the  never-ending  motion  of  water  all 'over  the  earth  in  seas, 
lakes,  rivers,  and  the  millions  of  little  streamlets,  diffuses  life  and  beHuty 
over  the  vegetable  world,  and  gives  to  man  the  vast  resources  which  we 
see  developed  in  the  numberless  applications  of  water-power  and  naviga- 
tion. 

Heat  is  everywhere  uniting  its  influence  with  the  other  forces  to  cause 
motion.  It  is  heat  that  produces  all  the  motions  of  the  air,  termed  winds. 
It  is  heat  that  causes  the  rise  of  the  water  all  over  the  earth  in  evaporation, 
so  that  it  may  be  collected  in  clouds,  again  to  descend  to  moisten  the  earth 


96  NATURAL  PHILOSOPHY. 

and  keep  the  ever-flowing  rivers  full.  Heat  applied  to  water  gives  to  man 
one  of  his  best  means  of  producing  motion  in  machinery. 

Light  and  electricity  are  also  manifestations  of  this  universal  force,  as 
will  be  shown  in  Chapters  XVII.  and  XVIII. ;  these  are,  to  a  certain  ex- 
tent, productive  of  motion. 

The  agencies  which  Chemistry  reveals  to  us  are  ever  at  work  causing 
motion  among  the  particles  of  matter ;  and  though  they  generally  work  in 
silence,  they  sometimes  show  themselves  in  tremendous  explosions,  and  in 
convulsions  of  nature. 

Busy  life  is  everywhere  producing  motion,  more  especially  in  the  animal 
world.  It  gives  to  the  myriads  of  animals,  great  and  small,  that  swarm 
the  earth  not  only  the  power  of  moving  themselves,  but  also  the  power,  to 
some  extent,  of  moving  the  material  world  around  them. 

57.  Varieties  of  Motion. — The  different  kinds  of  motion 
have  received  distinguishing  names;  the  following  list  em- 
braces the  principal  varieties,  with  examples  taken  from 
familiar  sources : 

• 

Varieties  of  Motion.  Examples. 

Slow The  sun's  shadow. 

Swift Lightning. 

Straight A  stone  dropped  into  a  well. 

Curved The  path  of  a  stone  in  the  air. 

Uniform The  hands  of  a  clock. 

Variable Winds,  animal  motions,  etc. 

Accelerated Gradually  increasing  motion. 

Retarded Gradually  diminishing  motion. 

Whether  motion  is  slow  or  swift  is  altogether  a  relative 
matter;  a  boy  may  run  very  swiftly,  yet  he  moves  slowly 
compared  with  a  race-horse,  and  the  horse  in  turn  cannot 
compete  with  the  locomotive,  while  the  speed  of  the  latter 
is  as  nothing  compared  with  the  inconceivably  rapid  mo- 
tion of  electricity.  The  rate  of  motion  is  called  velocity,  and 
it  is  measured  by  the  space  traversed  in  a  given  time.  Ve- 
locities are  compared  by  reference  to  the  distance  travelled 
in  one  second,  taken  as  a  standard  of  time,  very  swift  ve- 


MOTIONS    OF   MATTER.  97 

locities  being  expressed  in  miles  per  second,  and  slower 
ones  in  feet  per  second ;  this  will  be  understood  by  exam- 
ining the  following  table : 

TABLE   OF   COMPARATIVE   VELOCITIES.*  Miles  ill 

one  second. 

Light 1 92,500 

Electricity not  less  than  200,000 

Electric  currents  in  telegraph  wires 1  2,000 

Feet  in 
one  second. 

Relative  motion  of  the  sun  in  space 205,920 

Mean  rate  of  the  earth's  centre  in  its  path  around  the  sun. . .  101,061 

Sound  traversing  solid  bodies 11 ,286 

A  24-pound  cannon-ball  (maximum) 2,450 

lline-ball  (maximum) 1,600 

A  point  at  the  surface  of  the  earth  under  the  equator 1 ,525 

Volcanic  stones  projected  from  Etna 1,250 

A  point  at  the  earth's  surface,  latitude  of  London 950 

The  most  violent  hurricane 146 

Flight  of  a  swallow 134 

A  hurricane 117 

Locomotive  running  65  miles  per  hour 95 

An  ordinary  race-horse 42 

Flight  of  a  crow 37 

A  brisk  wind 8G 

The  fastest  sailing  vessel 15 

A  carriage  travelling  six  miles  an  hour nearly  9 

A  man  walking 6 

Straight  motion  is  one  which  does  not  change  its  direc- 
tion at  any  point ;  curved  motion,  on  the  other  hand,  is  con- 
tinually changing  its  direction.  These  require  no  special 
explanations,  but  to  the  latter  we  shall  refer  again. 

When  a  moving  body  passes  over  equal  distances  in  each 
second  of  time,  it  is  said  to  have  a  uniform  motion.  Our 
standard  of  uniform  motion,  with  which  we  compare  and 

*  Condensed  from  Arnott's  "Elements  of  Physics,"  Seventh  Edition. 


98  NATURAL   PHILOSOPHY. 

measure  all  other  motions,  is  that  of  the  earth  round  its  own 
axis.  "Here  we  have  a  huge  spinning-top,  which,  not  for 
hours  or  days,  but  for  unknown  ages,  has  kept  up  its  orig- 
inal speed  practically  undirninished.  All  our  notions  of 
time  are  based  on  the  regularity  with  which  the  earth  turns 
round." 

Watches  and  clocks  are  contrivances  for  obtaining  a  uni- 
form motion  which  can  be  compared  with  that  of  the  earth, 
and  for  marking  off  smaller  intervals  than  can  be  conven- 
iently observed  in  the  revolution  of  the  earth.  There  are 
very  few  cases  of  uniform  motion  in  the  world,  other  forces 
than  that  which  started  the  uniform  motion  constantly  re- 
tarding or  otherwise  modifying  it.  Motion  which  is  not 
uniform  or  regular  is  said  to  be  variable.  In  determining 
the  velocity  of  a  body  having  a  variable  motion,  we  must 
observe  the  rate  of  motion  at  various  equal  intervals  of 
time,  and  average  them.  The  distance  traversed  by  a  sail- 
ing vessel  or  steamer  is  ascertained  by  frequently  "  throw- 
ing the  log,"  by  means  of  which  the  speed  at  definite  times 
is  obtained,  and  calculating  the  average  velocity. 

When  a  moving  body  passes  over  gradually  diminishing 
distances  in  equal  intervals  of  time,  its  motion  is  said  to  be 
retarded.  Examples  of  uniformly  retarded  motion  are  fa- 
miliar: a  ball  rolled  along  the  ground  moves  more  and  more 
slowly  under  the  influence  of  gravitation  and  the  resistance 
offered  by  the  air  until  it  finally  comes  to  rest.  A  train 
detached  from  a  locomotive  has  its  motion  uniformly  re- 
tarded owing  to  the  same  causes ;  should  brakes  be  applied 
and  then  suddenly  released,  its  motion  would  also  be  re- 
tarded, but  not  uniformly.  Opposed  to  retarded  motion  is 
accelerated  motion,  in  which  the  velocity  of  a  body  contin- 
ually increases  until  external  forces  bring  it  to  rest;  as,  for 
example,  when  a  stone  is  dropped  from  a  height,  it  falls  1G 
feet  in  the  first  second,  04  feet  in  the  next  second,  144  feet 


MOTIONS    OF   MATTER.  99 

in  the  third  second,  and  so  on,  the  motion  being  uniformly 
\   accelerated,  owing  to  the  action  of  gravity. 

\      58.  Motion  and  Rest. — Though  we  use  the  term  rest  in 
>\ 

opposition  to  motion,  it  is  obvious  from  some  of  the  illus- 
trations given  that  rest  is  only  a  relative  term,  for  not  a 
particle  of  matter  in  the  universe  is  at  rest.  When  we  are 
sitting  still  we  call  ourselves  at  rest,  though  we  are  moving 
every  hour,  by  the  revolution  of  the  earth  on  its  axis,  1000 
miles  eastward,  and  68,000  miles  in  our  annual  journey  round 
the  sun.  Why,  then,  are  we  so  insensible  to  these  rapid 
motions  ?  It  is  partly  because  the  motions  are  so  uniform, 
but  chiefly  because  all  things  around  us,  our  houses,  trees, 
and  even  the  atmosphere,  are  moving  along  with  us.  If 
we  were  moving  along  alone,  even  at  a  slow  rate,  while  all 
these  objects  were  standing  still,  we  should  be  conscious  of 
our  motion,  as  when  we  ride  along  in  a  carriage  objects  at 
the  roadside  do  not  appear  to  move  along  with  us. 

This  can  be  made  more  clear  and  impressive  bj  a  familiar  comparison. 
A  man  on  board  of  a  steamboat,  by  confining  his  attention  to  things  within 
the  boat,  may,  after  a  while,  be  almost  unconscious  of  the  boat's  moving, 
if  the  water  be  smooth,  though  the  boat  may  be  going  at  the  rate  of  fifteen 
miles  an  hour.  If  he  be  reading  in  the  cabin,  he  will  think  as  little  of  his 
motion  as  he  would  were  he  reading  in  his  parlor  at  home.  Should  he  be 
blindfolded,  and  turned  around  a  few  times,  it  would  be  impossible  for  him 
to  tell  the  direction  in  which  the  boat  is  going.  Now  the  case  is  similar 
with  a  man  on  the  earth — he  is  unconscious  of  the  motion  of  the  earth  for 
the  same  reason  that  the  man  in  the  boat  is  unconscious  of  the  boat's  mo- 
tion. All  objects  around  him  are  moving  along  with  him,  as  the  objects 
around  the  man  in  the  cabin  of  the  boat  are  moving  along  with  him.  We 
can  carry  the  parallel  farther.  While  the  man  sits  in  the  cabin  he  knows 
not  how  fast  the  boat  moves,  nor  even  whether  it  moves  at  all.  He  must 
look  out  to  decide  this,  and  even  then  he  may  not  be  able  to  tell  whether 
the  boat  moves,  or  whether  he  merely  sees  the  water  running  by  it.  We 
are  often  actually  deceived  in  this  respect.  A  steamboat  struggling  against 
wind  and  wave  may  appear  to  those  on  board  to  be  advancing  when  it  is 
really  stationary,  or  even  when  it  is  losing  ground.  So  when  we  look  at 


100  NATURAL  PHILOSOPHY. 

the  sun,  we  know  not  whether  it  is  the  sun  or  the  earth  that  is  moving. 
Mere  vision,  without  reasoning  on  the  subject,  leads  one  to  think  that  it 
is  the  sun  that  moves. 

59.  Absolute  and  Relative   Motion,  —  The  motion  of  a 
body  is  said  to  be  absolute  when  it  is  considered  without 
relation  to  the  position  of  any  other  body.     Its  motion 
is  said  to  be  relative  when  it  is  moving  with  respect  to 
some  other  body.     Absolute  rest  is  unknown,  for  no  spot 
in  the  universe  is  known  to  be  without  motion.     But  a 
body  may  be  relatively  at  rest,  that  is,  in  a  fixed  relative 
position  to  other  bodies.     Every  body  is  in  a  state  of  abso- 
lute motion,  and  yet  it  may  be  in  a  state  of  relative  rest. 
All  objects  that  appear  to  us  to  be  at  rest  have  a  very 
rapid  absolute  motion.     They  appear  to  be  at  rest  merely 
because  they  have  the  same  rapidity  and  direction  of  abso- 
lute motion  that  we  have  ourselves.    And  all  the  motions 
which  are  apparent  to  the  eye  are  only  slight  differences 
in  the  common  absolute  motions,  of  which,  though  they  are 
so  exceedingly  rapid,  we  are  entirely  unconscious.     Thus, 
if  you  stand  still,  and  another  at  your  side  walk  at  the 
rate  of  three  miles  an  hour  eastward,  you  both  have  a  com- 
mon absolute  motion  of  1000  miles  in  every  hour,  and  he 
merely  adds  three  miles  to  his  thousand — you  move  1000 
miles,  and  he  1003.     So  if  you  sit  still  in  your  parlor,  and 
your  friend  travel  eastward  at  the  rate  of  20  miles  an 
hour,  you  move  every  hour  1000  miles,  and  he  1020.     And 
if  he  travel  westward  at  this  rate  he  really  travels  more 
slowly  than  you  do — he  has*  an  absolute  motion  eastward 
of  980  miles,  while  you  move  1000.     At  the  same  time  you 
are  both  whirling  on  in  the  annual  journey  around  the  sun 
at  the  rate  of  68,000  miles  an  hour. 

60.  Compound  Motion. — From  what  has  been  shown  in 
the  preceding  section,  it  is  evident  that  a  body  may  partake 
of  two  motions  at  one  and  the  same  time,  and  these  motions 


MOTIONS  '0V  M$iWBiJ   '••*'  5%  *  101 


may  be  in  the  same  direction  or  in  different  directions.  If 
a  man  travelling  on  a  steamboat  walk  towards  the  bow,  he 
will  move  forward  with  the  boat  at  the  same  time;  sup- 
pose the  steamboat  moves  at  seven  miles  an  hour,  and  he 
walks  at  the  rate  of  three  miles  an  hour,  during  the  time 
that  he  passes  from  the  stern  to  the  bow  of  the  boat  his 
total  velocity  will  be  ten  miles  per  hour  ;  if,  on  the  other 
hand,  he  turn  about  and  walk  back  to  the  stern  at  the  same 
rate,  his  velocity  will  be  only  four  miles  an  hour.  That  is, 
if  we  refer  his  motion  to  the  banks  of  the  river  on  which 
the  steamboat  moves  ;  but  if  we  refer  his  motion  to  the 
steamboat  only,  his  velocity  will  be  three  miles  an  hour,  no 
matter  in  what  direction  he  walks.  This  is  an  example  of 
simultaneous  motions  in  the  same  direction  ;  we  will  now 
give  one  of  simultaneous  motions  in  different  directions. 

If  a  man  attempt  to  row  a  boat  straight  across  a  swift- 
ly running  river,  he  will  reach  a  point  not  directly  opposite 
to  that  from  which  he  started,  but  below.  Two  forces  act 
upon  the  boat:  the  current  tending  to  carry  it  straight 
down  the  stream,  and  his  rowing  tending  to  carry  it 
straight  across.  The  boat  will  go  in  neither  of  these 
directions,  but  in  a  line  between  them. 
Let  A  B,  Fig.  72,  represent  the  bank  of 
the  river,  from  which  he  starts  at  A, 
with  the  bow  of  the  boat  pointing  to  C, 
on  the  opposite  bank.  Suppose,  now,  Flg-  72> 

that  in  the  time  that  it  takes  him  to  royr  across  the  cur- 
rent would  carry  him  down  to  B  if  he  did  not  row  at  all. 
He  will  in  this  time,  by  the  two  forces  together,  reach  the 
point  D,  opposite  to  B,  his  course  being  the  line  A  D.  If 
the  wind  blow  upon  a  vessel  in  such  a  way  as  to  carry 
it  eastward,  and  a  current  be  pushing  it  southward,  the 
vessel  will  run  in  a  middle  line,  viz.,  southeast.  For  the 
same  reason,  if  a  boy  kick  a  foot-ball  already  in  motion,  it 

E 


102 


PHILOSOPHY. 


will  not  be  carried  in  the  direction  in  which  he  kicks  it, 
but  in  a  line  between  that  direction  and  the  direction  in 
which  its  former  motion  was  carrying  it.  In  swimming, 
flying,  rowing,  etc.,  we  have  examples  of  compound  mo- 
tion, the  middle  line  between  the  directions  of  the  forces 
always  being  taken  by  the  body  moved. 

If  we  take  Fig.  72,  illustrating  the  movement  of  the  boat, 

c, ,,D  and  draw  two  lines,  one  from  A  to  C  and 

the  other  from  B  to  D,  we  shall  have  the 
parallelogram  A  C  D  B,  Fig.  73,  in  which 
B  the  line  A  C  represents  the  force  of  the 
Fig.  73.  rowing,  A  B  the  force  of  the  current,  and 

A  D  the  path  of  the  boat.  You  see,  then,  that  if  we  wish 
to  find  in  what  direction  and  how  far  in  a  given  time  a 
body  acted  upon  by  two  forces  will  move,  we  are  to  draw 
two  lines  in  the  direction  of  these  forces,  and  of  a  length 
proportionate  to  the  distances  to  which  they  would  move 
it  in  that  time ;  then  by  drawing  two  lines  parallel  to 
these  we  shall  have  a  parallelogram,  and  the  diagonal  of 
this  will  represent  the  distance  and  the  course  of  the 
moving  body.  If  a  body  be  acted  upon 
by  two  equal  forces  and  at  right  angles 
to  each  other,  the  figure  described  will 
be  a  square,  as  you  see  in  Fig.  74.  If 
they  vary  from  a  right  angle,  the  figure 
will  vary  in  the  same  proportion  from 
the  square  figure,  as  seen  in  Figs.  75  Fig.  74. 

and  76.  In  the  three  figures,  A  B 
and  A  D  represent  the  two  forces 
and  A  C  the  resulting  motion. 
You  observe  by  these  diagrams 
that  the  nearer  the  two  forces  are 
c  to  the  same  direction,  the  farther 
will  they  move  the  body.  This 


Fig.  75. 


MOTIONS    OF   MATTER.  103 

is   shown  by  the   different 
leno-ths  of  the  diagonals  in 

O  O 

Fig.  74  and  Fig.  76.  The 
more  nearly,  therefore,  the 
Avind  coincides  with  the  current,  the  more  rapidly  will  a 
vessel  be  carried  along  before  the  wind.  When,  on  the 
other  hand,  the  angle  at  which  two  forces  act  upon  a  body 
is  much  greater  than  a  right  angle,  they  will  propel  it  but 
a  small  distance.  Thus,  if  two  forces  act  on  a  body  at  D 
in  the  directions  D  A  and  D  0,  Fig.  77,  they  will  move  it 


only  the  distance  represented  by  the  diagonal  D  B.  This 
diagram  represents  the  motion  of  a  vessel  sailing  almost 
directly  against  a  current  by  a  wind  the  force  of  which  is 
equal  to  that  of  the  current,  while  Fig.  76  represents  the 
motion  of  a  vessel  where  wind  and  current,  being  of  equal 
force,  very  nearly  coincide.  In  the  above  diagrams  we 
have  supposed  the  forces  to  be  equal-;  but  the  same  truth 
can  be  shown  in  regard  to  unequal  forces. 

61.  Momentum. — The  momentum  of  a  body  is  its  quan- 
tity of  motion.  In  estimating  the  momentum  of  any  body 
two  things  must  be  considered — its  velocity,  and  its  quan- 
tity of  matter  or  weight.  *  A  bullet  fired  from  a  gun  has  a 
vastly  greater  force,  or  power  of  overcoming  obstacles,  than 
one  thrown  by  the  hand,  owing  to  its  greater  velocity. 
Now,  suppose  the  weight  or  quantity  of  matter  to  be  in- 
creased ten  times,  and  that  it  moves  with  the  same  velocity 
as  before;  it  will  have  ten  times  as  much  force  as  before, 
and  will  overcome  ten  times  as  great  an  obstacle.  For  this 
reason,  a  small  stone  dropping  upon  a  man's  head  may  do 


104 


NATURAL   PHILOSOPHY. 


but  little  harm,  while  one  ten  times  as  large,  falling  from 
the  same  height,  may  stun  and  perhaps  kill  him.  But  if  the 
large  stone  could  fall  with  only  one  tenth  of  the  velocity 
of  the  small  one,  the  effect  of  both  would  be  the  same. 
The  rule  for  calculating  the  momentum  of  a  moving  body 
is  to  multiply  its  weight  by  its  velocity.  Using  the  above 
illustration  for  an  example,  suppose  the  weight  of  the  small 
stone  be  1  ounce,  and  that  of  the  large  one  10  ounces.  If 
they  fall  from  a  height  of  16  feet,  the  force  with  which  thu 
large  one  will  strike  will  be  expressed  by  160  (16  x  10),  that 
of  the  small  one  by  16  (1  x  16).  Suppose,  however,  that 
by  some  force  in  addition  to  gravity  the  small  one  could 
be  made  to  move  ten  times  as  fast  as  the  large  one,  the 
force  with  which  it  would  strike  would  be  equal  to  that  of 
the  large  one,  and  would  be  expressed  by  the  number  160. 
We  will  illustrate  this  in  another  way.  Let  a  and  £,  Fig. 
78,  be  two  balls  of  clay  of  equal  size 
hanging  over  a  graduated  arc.  Now 
if  b  be  let  fall  from  the  top  of  the  arc 
6,  on  striking  against  a  it  gives  half 
of  its  motion  to  a,  and  they  both 
move  on  together.  But  how  far  will 
they  go  ?  To  3  on  the  other  side 
of  the  arc.  Why?  Let  the  quan- 
tity of  matter  in  each  ball  be  called 
1,  and  the  motion  of  b  6.  The  momentum  will,  therefore, 
be  6.  Now  the  momentum  of  the  two  together  will  be  the 
same  after  the  blow  as  that  of  b  was  before  it.  But  the 
quantity  of  matter  is  twice  as  great,  and  must  be  called  2. 
Therefore  the  motion  must  be  represented  as  3,  to  make  the 
momentum  6  (2  x  3).  But  suppose  that  b  is  twice  as  large  as 
a.  Falling  from  6,  its  momentum  would  be  represented  by 
12  (2  x6).  After  it  has  struck  «,  the  momentum  of  the  two 
together  would  be  the  same  as  that  of  b  before  the  stroke ; 


MOTIONS    OF   MATTER.  105 

but  the  quantity  being  3,  the  motion  would  be  represented 
by  4.     They  would,  therefore,  move  to  4  on  the  arc. 

Examines. — A  few  examples  illustrating  momentum,  as  dependent  upon 
weight  and  velocity,  will  suffice.  If  a  musket-ball  of  an  ounce  weight  were 
so  far  spent  as  to  move  with  a  velocity  of  only  a  foot  in  a  second,  its 
force  would  be  so  small  that  if  it  hit  any  one  it  would  do  little  harm.  But 
a  cannon-ball  weighing  a  thousand  ounces  moving  at  this  slow  rate  would 
have  a  very  great  force — equal,  in  fact,  to  the  momentum  of  an  ounce  ball 
moving  1000  feet  in  a  second. — If  a  plank  push  a  man's  foot  against  a 
wharf,  he  will  scarcely  feel  it ;  but  if  the  plank,  instead  of  being  alone,  is 
one  of  a  thousand  planks  fastened  together  in  a  raft,  and  the  whole  move 
with  the  same  velocity,  the  force  will  be  increased  a  thousand-fold,  and  the 
plank  will  crush  the  foot.  And  if  the  one  plank,  when  alone,  should  move 
a  thousand  times  as  fast  as  the  whole  raft,  the  same  result  would  follow. — 
So  soft  a  substance  as  a  candle  can  be  fired  through  a  board  from  the  mo- 
mentum given  to  it  by  an  immense  velocity. — Perhaps  there  is  no  better 
example  of  the  great  force  given  to  a  substance  by  an  enormous  velocity 
than  we  have  in  the  wind.  So  light  a  thing  is  air  that  people  think  of  it 
as  almost  nothing.  But  let  it  be  set  in  rapid  motion,  and  the  velocity  gives 
to  it  a  force,  a  momentum,  which  will  drive  ships  upon  the  shore,  throw 
over  buildings,  and  tear  up  trees  by  the  roots.  In  this  last  example  we 
see  beautifully  illustrated  the  meaning  of  the  expression  quantity  of  mo- 
tion. In  the  moving  air  each  particle  does  its  share  of  the  work  in  the  de- 
structive effects  mentioned.  Each  particle,  therefore,  may  be  considered 
as  a  reservoir  of  motion,  and  the  quantity  of  motion  in  any  case  depends 
upon  the  quantity  which  each  particle  has  and  the  number  of  the  particles. 

62.  Relation  of  Force  to  Velocity. — It  would  seem,  at  first 
thought,  that  the  motion  produced  in  any  body  must  be  in 
exact  proportion  to  the  force  producing  it;  that  is,  that 
twice  the  force  which  produces  a  giv-en  velocity  would 
double  that  velocity,  and  three  times  would  treble  it,  etc. 
This  is  true  where  there  is  no  resistance  to  motion,  as  in 
the  case  of  the  heavenly  bodies  moving  in  their  orbits. 
But  in  all  motions  here  upon  the  earth  there  is  resistance ; 
and  the  greater  the  velocity,  the  greater  the  resistance.  If, 
therefore,  you  increase  the  velocity  of  any  body,  you  not 
only  have  to  communicate  more  motion  to  it,  but  you  must 


106  NATURAL   PHILOSOPHY. 

overcome  also  the  increased  resistance.  The  rate  of  in- 
crease of  force  for  increased  velocities  has  been  very  accu- 
rately ascertained.  A  boat  moving  from  B  to  A,  Fig.  79, 
we  will  suppose,  displaces  a  quantity  of  water  represented 


Fig.  79. 

by  the  space  between  the  two  lines  extending  from  B  to  A. 
Now  if  it  move  from  B  to  C,  it  displaces  twice  the  bulk  of 
water  B  C ;  and  as  it  is  displaced  in  the  same  time  that  B 
A  was,  each  particle  is  displaced  with  twice  the  velocity. 
Double  the  force  is  required  to  displace  a  double  portion 
of  water,  and  to  do  this  with  double  the  velocity  the  force 
must  be  doubled  again.  So  if  the  boat  be  made  to  move 
three  times  as  far  in  the  same  time — that  is,  from  B  to  D — 
three  times  the  quantity  of  water  is  displaced,  and  each  of 
these  three  portions,  B  A,  A  C,  and  C  D,  is  displaced  with 
three  times  the  velocity.  The  force  required,  then,  to  do 
this  is  nine  times  that  required  to  carry  the  boat  from  B  to 
A  in  the  same  time.  It  is  plain,  therefore,  that  with  veloc- 
ities represented  by 'the  numbers  1,  2,  3,  4,  etc.,  the  forces 
requisite  to  produce  these  velocities  must  be  as  the  squares 
of  these  numbers;  viz.,  1,  4,  9,  16,  etc.  This  law  is  a  very 
important  one,  in  a  practical  point  of  view.  For  example, 
it  shows  us  how  much  larger  a  quantity  of  coal  is  required 
to  produce  in  steamboats  a  high  velocity  than  a  moderate 
one.  Its  application,  too,  to  the  science  of  gunnery  is  im- 
portant. 

When  the  weight  of  a  moving  body  is  multiplied  by  its 
velocity,  we  obtain  (§  61)  its  momentum;  when  the  weight 
is  multiplied  by  the  square  of  the  velocity,  we  obtain  the 
force  with  which  a  body  strikes  a  resisting  substance.  This 
is  directly  deduced  from  the  explanation  just  given. 

63.  Accelerated  Force.— You  have  learned  in  §  57  some- 


MOTIONS    OP    MATTER.  107 

thing  of  the  varieties  of  motion:  these  are  obviously  the  re- 
sult of  the  action  of  corresponding  forces  on  matter.  Thus 
we  have  uniform,  accelerated,  and  retarded  forces.  If  the 
momentum  remain  the  same,  independently  of  time,  the 
force  is  uniform;  if  the  momentum  increase,  the  force  is. ac- 
celerated ;  and  if  diminished,  the  force  is  retarded.  Were 
there  no  obstacles  to  motion,  such  as  resistance  of  the  air, 
etc.,  we  might  have  uniform  forces;  but  since  we  do  not 
meet  with  absolutely  isolated  or  free  matter,  all  moving 
forces  are  more  or  less  variable.  Even  in  the  production 
of  very  rapid  motions  the  force  is  seldom  instantaneously 
applied,  but  is  rather  gradual  in  its  action ;  the  motion  is 
not  the  result  of  a  single  impulse,  but  a  succession  of  im- 
pulses is  required  to  accumulate  sufficient  momentum  to 
overcome  the  resistance  opposed.  The  action  of  gunpow- 
der upon  a  bullet  issuing  from  a  gun  is  apparently  an  in- 
stantaneous and  single  impulse,  but  it  is  not  really  so.  The 
great  velocity  given  to  the  bullet  is  due  to  the  continued 
impulse  of  the  expansive  force  of  gases  produced  from  the 
powder,  and  it  therefore  depends  much  on  the  length  of  the 
barrel.  If  this  be  short,  the  force  of  the  powder  is  not  con- 
fined long  enough  to  the  bullet  to  give  it  a  great  velocity. 

It  is  on  the  same  principle  of  continued  action  that  a  man  lifts  his  ham- 
mer high  when  he  wishes  to  inflict  a  heavy  blow.  In  this  case  both  grav- 
itation and  the  muscular  power  of  the  arm  exert  their  force  on  the  hammer 
through  the  whole  space.  A  horse  in  kicking  does  the  same  thing,  and  by 
the  great  length  of  the  leg  the  velocity  given  to  the  foot  by  this  continued 
action  of  the  muscles  is  very  great.  An  arrow  is  not  shot  by  a  single  mo- 
mentary impulse  of  the  bowstring,  but  the  string,  by  following  it  through 
a  considerable  space,  gives  it  a  continued  impulse. 

One  of  the  best  examples  of  accelerated  force  is  the  attraction  of  gravity. 
You  know  that  the  greater  the  elevation  from  which  a  body  falls,  the 
greater  is  its  velocity,  and,  therefore,  the  greater  the  force  with  which  it 
strikes.  Why  is  this  ?  If  it  fell  because  of  a  single  impulse  drawing  it 
towards  the  earth,  this  would  not  be  the  case ;  and  if  there  were  no  air 


108  NATURAL   PHILOSOPHY. 

in  the  way,  the  velocity  would  be  uniform.  But  the  resistance  of  the  air 
would  retard  the  velocity ;  so  that  if  a  number  of  bodies  should  receive 
the  same  impulse  at  different  elevations,  the  one  farthest  off  would  be 
most  retarded,  and,  therefore,  come  down  slower  than  all  the  rest.  In  this 
case,  the  higher  the  elevation  from  which  a  man  should  fall,  the  less  would 
be  the  injury.  But  a  body  does  not  come  to  the  ground  by  a  single  im- 
pulse, but  by  a  succession  of  impulses,  or,  rather,  a  continued  impulse.  Ev- 
ery moment  that  the  body  is  coming  down  it  is  drawn  upon  by  the  attrac- 
tion of  the  earth,  and  this  continued  action  causes  an  increase  in  the  rapid- 
ity of  motion. 

Expressing  this  in  somewhat  different  language,  we  may 
say  that  gravity  is  an  accelerating  force.  Of  this  examples 
are  innumerable:  "A  person  may  leap  from  a  chair  with 
impunity;  if  from  a  table,  he  receives  a  harder  shock;  if 
from  a  high  window,  a  topmast  of  a  ship,  or  the  parapet  of 
a  high  bridge,  he  will  probably  fracture  bones;  and  if  he 
fall  from  a  balloon  at  a  great  height,  his  body  will  be  lit- 
erally dashed  to  pieces." 

Water  falling  from  a  height  acquires  a  power  proportion- 
al to  the  elevation.  The  same  is  true  of  meteoric  stones, 
which  approach  the  earth  with  such  immensely  accelerated 
velocity  that  they  become  heated  in  their  passage  through 
the  atmosphere,  and  bury  themselves  deep  in  the  earth 
when  they  strike  its  surface. 

64.  Gravity  a  Uniformly  Accelerating  Force. — An  acceler- 
ating force  may  be  uniform  or  variable;  gravity  is  not  only 
an  accelerating  force,  but  it  is  uniform  in  its  rate  of  increase. 

A  stone  dropped  from  a  height  falls  through  a  distance 
of  16  feet  in  one  second,  64  feet  in  two  seconds,  144  feet  in 
three  seconds,  and  so  on.  Now,  after  the  stone  has  passed 
through  the  16  feet — that  is  to  say,  at  the  end  of  the  first 
second  of  time,  its  velocity  is  32  feet  per  second ;  at  the  end 
of  two  seconds,  64  feet  per  second  ;  at  the  end  of  three  sec- 
onds, 96  feet  per  second,  and  so  on.  Thus  the  velocity  at 
the  end  of  2,  3,  4  seconds  is  double,  triple,  quadruple,  etc., 


MOTIONS    OF   MATTER. 


109 


that  at  the  end  of  one  second ;  that  is,  its  rate  of  increase 
is  uniform,  viz.,  32  feet  per  second. 

This  law  holds  good  for  all  bodies,  no  matter  whether 
they  are  heavy  or  light.  At  first  sight  it  seems  very  par- 
adoxical that  a  ball  weighing  one  pound  dropped  from  a 
height  will  reach  the  ground  just  as  quickly  as  one  weigh- 
ing ten  times  or  one  thousand  times  as  much.  And  yet 
such  is  the  case ;  gravity  causes  all  unimpeded  bodies  to  fall 
with  equal  rapidity,  without  reference  to  their  weight.  Any 
one  who  throws  a  feather  and  a  bullet  into  the  air,  however, 
observes  tl^at  the  bullet  falls  to  the  ground  long  before  the 
feather,  and  will  be  disposed  to  dispute  the  statement  just 
made.  Such  a  one  must  remember  that  the  resistance  of 
the  air  must  be  taken  into  account,  as  we  will 
now  proceed  to  show. 

When  a  stone  is  thrown  into  the  air,  its  upward  motion  is 
gradually  destroyed  by  the  attraction  of  the  earth  and  the  re- 
sistance of  the  air.  Observe,  now,  why  it  descends.  It  is 
from  the  action  of  one  of  the  causes  which  arrested  its  upward 
flight  —  the  attraction  of  the  earth.  In  its  descent  it  is  re- 
tarded by  the  resistance  of  the  air,  as  it  was  in  its  ascent. 
This  retardation  is  very  obvious  in  the  case  of  substances 
which  present  a  large  surface  to  the  air,  as  a  feather.  A 
small  piece  of  lead  will  outweigh  many  feathers,  and,  therefore, 
since  its  quantity  of  matter  is  so  much  greater  in  proportion 
to  its  surface  than  that  of  a  feather,  it  will  fall  to  the  ground 
much  more  quickly.  That  this  is  owing  wholly  to  the  resist- 
ance of  the  air  can  be  proved  with  the  air-pump.  Suppose 
that  you  have  a  tall  receiver,  Fig.  80,  on  the  air-pump,  and  a 
piece  of  lead  and  a  feather  are  placed  at  its  upper  part  in  such 
a  way  that  they  can  be  made  to  fall  at  the  same  instant.  Ex- 
haust the  air,  and  then  let  them  fall.  They  will  go  down  side 
by  side,  as  represented  by  the  figure,  and  reach  the  bottom  of 
the  receiver  at  the  same  time,  because  there  is  no  air  to  resist 
the  progress  of  the  feather. 

The  toy  called  the  water-hammer  illustrates  the  same  thing. 
When  water  falls  through  the  air,  the  resistance  of  the  air  tends 

E2 


110  NATUEAL  PHILOSOPHY. 

to  separate  its  particles,  as  we  see  in  the  falling  of  water  thrown  up  by  a 
fountain.  In  the  water-hammer,  which  is  a  closed  tube  containing  a  little 
water  and  no  air,  when  the  water  is  made  to  fall  from  one  end  to  the  other, 
as  there  is  no  air  to  divide  it,  it  falls  as  one  mass,  and  gives  a  sharp  sound 
like  the  blow  of  a  hammer.  An  instrument  essentially  like  this  can  be  made 
with  a  thin  glass  flask.  Put  a  little  water  into  it,  and,  after  heating  it  to 
boiling  over  a  spirit-lamp,  cork  the  flask  tightly,  and  then  leave  the  water 
to  cool.  As  all  the  space  above  the  water  was  filled  with  steam  when  the 
flask  was  corked,  it  is  a  vacuum  now  that  the  steam  is  condensed. 

65.  Retarded  Force. — We  have  seen  (§  63)  that  force  is 
never  instantaneously  communicated,  and  that  a  succession 
of  impulses  are  required  to  communicate  motion.  In  like 
manner,  no  force  can  be  instantaneously  arrested,  and  a 
gradual  resistance  to  motion  is  necessary  to  make  it  dis- 
appear. Examples  showing  the  gradual  nature  of  the  re- 
tardation of  force  are  numerous.  It  is  by  the  gradual  or 
continued  resistance  of  the  air  that  the  motion  of  a  cannon- 
ball  is  destroyed.  Now  if,  instead  of  this  gradual  resist- 
ance, any  hard  substance,  as  a  block  of  granite,  were  op- 
posed to  the  progress  of  the  ball,  it  would  be  at  once 
broken  asunder.  We  see,  then,  the  reason  that  a  hard  sub- 
stance of  moderate  thickness  does  not  offer  so  effectual  a 
resistance  to  a  body  moving  very  rapidly  as  some  sub- 
stance of  a  more  yielding  kind  and  of  greater  bulk.  For 
example,  a  bale  of  cotton  will  arrest  a  ball  which  would 
pass  through  a  plank,  for  the  cotton,  yielding  easily,  permits 
the  force  of  the  ball  to  be  felt  and  resisted  by  a  larger 
bulk,  while  the  wood,  not  yielding,  opposes  but  a  small 
portion  of  its  whole  bulk  to  the  force  of  the  ball,  and 
therefore  does  not  arrest  it ;  in  other  words,  the  momentum 
of  the  ball  is  communicated  to  a  much  larger  quantity  of 
matter  in  the  cotton  than  in  the  wood.  These  principles 
afford  a  ready  explanation  of  a  feat  which  is  sometimes 
performed.  A  man  lies  upon  his  back,  and,  having  an 
anvil  carefully  placed  upon  his  chest,  allows  some  one  to 


MOTIONS    OF   MATTER.  Ill 

strike  a  heavy  blow  with  a  hammer  upon  the  anvil,  and 
no  injury  is  received.  Why?  Because  the  momentum,  or 
force,  of  the  hammer  is  diffused  throughout  the  bulk  of  the 
anvil,  and  then  again  throughout  the  bulk  of  the  yielding 
chest.  The  man  takes  good  care  to  have  his  lungs  well 
filled  with  air  at  the  moment  of  the  blow,  for  this  increases 
the  bulk  and  elasticity  of  the  chest,  and  thus  promotes  the 
diffusion  of  the  momentum.  If  the  blow  of  the  hammer 
were  received  directly  upon  the  chest,  great  injury  would 
be  done,  for  the  force  would  then  be  spent  upon  one  small 
spot  alone. 

The  principles  above  elucidated  are  applied  by  men  instinctively  in  their 
common  labors  and  efforts.  Watch  a  man  catching  bricks  that  are  tossed 
to  him.  As  he  receives  the  bricks  in  his  hands  he  lets  his  hands  and  the 
bricks  move  together  a  little  way,  so  that  he  may  gradually  arrest  the  mo- 
tion of  the  bricks.  To  do  it  suddenly  would  give  him  a  painful  lesson  on 
momentum.  So  when  a  man  jumps  from  a  height  he  does  not  come  to  the 
ground  in  a  straight  position.  This  would  cause  a  sudden  and  therefore 
a  painful  arrest  of  the  motion  of  the  whole  body.  To  avoid  this  he  comes 
to  his  feet  with  all  the  great  joints  of  his  body  bent,  so  that  the  different 
portions  approach  the  ground  successively,  his  head  having  its  motion 
arrested  last. 

QUESTIONS. 

55.  Explain  the  relations  of  matter,  motion,  and  force  as  seen  in  the 
rolling  of  a  ball.  Define  motion.  Define  force.  Show  that  force  may 
exist  without  motion.  Give  the  illustration  of  the  magnet. — 56.  What 
is  said  of  the  universality  of  motion  ?  What  are  the  principal  causes  of 
motion?  How  does  attraction  act?  Explain  the*  influence  of  heat  and 
of  other  forces. — 57.  Name  the  varieties  of  motion,  and  give  examples  of 
each.  How  is  velocity  measured  ?  How  compared  ?  Name  the  rate  of 
motion  of  light.  Of  electricity.  Of  sound.  Of  a  violent  hurricane.  Of 
a  man  walking.  Illustrate  uniform  motion.  Illustrate  variable  motion. 
Give  examples  of  uniformly  retarded  motion.  Of  accelerated  motion. 
— 58.  Show  that  motion  and  rest  are  relative  terms.  Give  the  comparison 
of  the  steamboat. — 59.  What  is  the  difference  between  absolute  and  rel- 
ative motion?  What  is  said  of  absolute  rest  ?— GO.  What  is  said  of  com- 


112  NATURAL   PHILOSOPHY. 

pound  motion.  Give  the  example  of  a  man  walking  the  deck  of  a  moving 
steamboat.  Illustrate  simultaneous  motions  in  different  directions.  Ex- 
plain the  principles  illustrated  by  the  parallelograms,  Figs.  73,  74,  and  75. 
— Gl.  What  is  the  momentum  of  a  body?  Give  the  rule  for  calculating 
momentum,  and  give  an  example.  Give  the  illustration  of  two  balls  of 
clay.  Also  of  the  cannon-ball.  And  of  the  plank.  What  is  said  of  the 
wind  as  a  reservoir  of  motion  ? — 62.  What  is  said  of  the  relation  of  force 
to  velocity  ?  —  63.  What  is  said  of  accelerated  force  ?  Show  that  rapid 
motions  are  usually  caused  by  a  succession  of  impulses.  Give  the  illustra- 
tions of  the  hammer,  of  the  horse,  and  of  the  arrow.  Show  that  gravity  is 
an  accelerating  force.  Give  illustrations. — 64.  Show  that  gravity  is  uni- 
formly accelerating.  State  the  law.  Explain  why  bodies  of  different 
weights  fall  with  equal  rapidity.  Describe  the  experiment  in  a  vacuum. 
Give  the  illustration  of  the  water-hammer. — 65.  What  is  said  of  retarded 
force?  Give  examples.  Explain  the  anvil  trick.  Mention  illustrations 
taken  from  every-day  life. 


CHAPTER  VIII. 

MOTIONS    OF   MATTER    (CONTINUED). 

66.  Course  of  Bodies  Thrown  into  the  Air. — When  any  body 
— a  stone,  for  example — is  thrown  straight  upward  into  the 
air,  it  does  not,  in  reality,  go  up  or  come  down  vertically. 
If  it  did,  it  would  come  down  at  a  great  distance  from  us. 
Suppose  it  takes  two  seconds  for  it  to  go  up  and  to  reach 
the  ground.  If  we  stand  at  the  equator,  in  that  two  sec- 
onds we  move  from  the  point  where  we  threw  up  the  stone 
nearly  3000  feet  eastward ;  and,  therefore,  if  the  stone  rose 
and  fell  vertically,  it  would  fall  3000  feet  westward  of  us. 
Why,  instead  of  this,  does  it  fall  at  our  feet?  Because 
when  thrown  into  the  air  it  not  only  has  the  upward  mo- 
tion given  by  the  hand,  but  also  the  forward  motion  of  the 
earth.  It  is  a  case  similar  to  that  of  a  man  on  board  of  a 
steamboat,  who,  though  the  vessel  move  fifteen  miles  an 


MOTIONS   OP   MATTEK.  113 

hour,  tosses  up  his  ball  or  orange  and  catches  it  as  well  as 
if  he  were  on  land.  This  he  could  not  do  if  both  he  and 
the  orange  did  not  have  the  same  forward  motion  as  the 
boat.  If  a  man  fall  from  a  mast-head,  he  reaches  the  deck 
at  the  foot  of  the  mast  when  the  vessel  is  sailing  rapidly, 
just  as  if  it  were  lying  still  at  the  wharf.  If  he  did  not 
by  inertia  (§  14)  retain  the  forward  motion  which  he  had 
in  common  with  the  vessel,  he  would  fall  at  some  distance 
behind  the  mast. 

The  Earth  and  the  Atmosphere. — The  air  being  held  to  the  earth  by  at- 
traction, it  has  a  motion  in  common  with  the  earth.  It  revolves  with 
the  earth  just  as  the  tire  of  a  wheel  revolves  with  the  wheel.  This  being 
so,  our  winds  are  nothing  but  slight  variations  of  this  constant  rapid  whirl 
of  the  aerial  coating  of  the  earth.  If  the  atmosphere  were  suddenly  to 
stop  whirling  round  with  the  earth,  we  should  move  through  it  with  a 
velocity  of  1500  feet  a  second;  and  the  destructive  effect  upon  us  would 
be  the  same  as  if  the  earth  were  standing  still  while  the  air  moved  over 
its  surface  with  this  fearful  velocity.  A  thoughtless  man,  not  reflecting 
that  the  atmosphere  moved  with  the  earth,  proposed  rising  in  a  balloon,  and 
waiting  till  the  country  to  which  he  wished  to  go  should  pass  under  him, 
and  then  to  descend  to  the  earth. 

67.  Path  of  Projectiles.  —  If  we  consider  the  connection 
between  the  motion  of  the  earth  and  the  course  of  a  body 
thrown  into  the  air,  and  ascertain  its  actual  path,  we  find 
that  it  forms  a  peculiar  curve. 

Anything  thrown  into  the  air  is  called  a  projectile  /  and 
the  path  which  it  follows  is 
that  of  &  parabola.  Suppose 
a  stone  be  thrown  by  a  man 
standing  at  A,  Fig.  81,  in  the 
direction  A  C  E  G,  it  will  de- 
viate from  a  straight  line 
on  account  of  the  attraction 
of  gravity,  and  actually  de- 
scribe  the  parabolic  curve 


114  NATURAL   PHILOSOPHY. 

A  D  F  B.  If  the  stone,  having  reached  the  point  C,  has 
fallen  towards  the  earth  a  certain  distance,  represented  by 
C  D,  when  it  reaches  the  point  E,  twice  as  far  from  A,  it 
will  fall  a  distance  not  twice,  "but,  four  times  as  great  as 
C  D,  viz.,  E  F,  and  so  on,  thus  forming  the  curve. 

Of  course,  it  is  understood  that,  besides  the  propelling 
force  of  the  arm  and  the  attraction  of  gravity,  a  third  force 
acts  upon  the  stone,  viz.,  the  resistance  of  the  air;  but  this 
being  in  direct  opposition  to  the  first,  it  only  retards  the 
motion,  and  does  not  tend  to  turn  it  from  its  straight 
course. 

If  the  stone  be  thrown  horizontally,  it  also  describes 
a  parabola.  If  the  propulsive  force  be  very  great,  as  in 
the  case  of  a  bullet  discharged  from  a  gun,  the  path  will 
appear  to  be  straight;  but  this  is  not  so.  The  force  of 
gravity  pulls  the  bullet  towards  the  ground  from  the  in- 
stant that  it  leaves  the  gun.  This  deviation  is  very  slight, 
however,  and  for  short  distances  the  bullet  may  be  con- 
sidered as  moving  in  a  straight  line.  When,  however,  a 
marksman  shoots  at  long  range,  he  must  make  allowance 
for  this  bending-down  of  the  motion.  Accordingly,  for  the 
sake  of  precision,  a  double  sight  is  provided  in  modern  guns, 
as  shown  at  A  and  B,  Fig.  82.  This  arrangement  secures 


Fig.  82. 


the  pointing  of  the  gun  a  little  above  the  level  of  the 
object  aimed  at,  that  level  being  indicated  by  the  dotted 
line. 

Let  us  further  study  the  path  of  a  projectile  impelled 


MOTIONS    OF   MATTER.  115 

horizontally.  In  the  case  of  the  musket-ball  just  men- 
tioned, we  have  seen  that  two  forces  act  upon  it,  viz.,  the 
projectile  force  given  by  the  powder  and  the  force  of  gravi- 
tation. The  force  of  gravity  being  always  the  same,  the 
shape  of  the  curve  which  the  projected  body  describes 
must  depend  on  the  force  with  which  it  is  projected.  This 
is  very  strikingly  exemplified  in  the  curves  described  by  the 
different  streams  of  water  in  Chapter  XII.  But  whether 
the  projectile  force  be  great  or  small,  the  moving  body 
thrown  horizontally  will,  in  every  case,  reach  the  ground  in 
the  same  time.  Thus,  if  two  cannons  stand  side  by  side  on 
a  height,  one  of  which  will  send  a  ball  a  mile  and  the  other 
half  a  mile,  the  two  balls,  if  fired  together,  will  reach  the 
ground  at  the  same  instant,  though  at  first  thought  it 
would  seem  that  the  ball  which  travels  twice  as  far  as  the 
other  would  take  a  longer  time  to  accomplish  it.  This  is 
because  the  horizontal  force  of  the  ball  does  not  oppose  in 
the  least  the  downward  force  of  gravity.  If  it  were  thrown 
upward  instead  of  horizontally,  the  projectile  force  wrould 
be  opposed  to  gravity,  and  in  proportion  as  the  direction 
came  near  to  being  vertical.  As  horizontal  force  does  not 
interfere  with  the  action  of  the  force  of  gravity,  it  follows 
that,  if  a  ball  be  dropped  at  the  instant  at  which  another 
is  fired,  both  will  reach  the  ground  at  the  same  instant. 
This  can  be  made  clear  by  Fig.  83.  Suppose  it  takes 
three  seconds  for  a  ball  to  fall  from  the  top  of  a  tower 
to  its  foot.  In  the  first  second  it  falls  to  a.  The  ball 
projected  horizontally  from  the  cannon,  being  operated 
upon  by  the  same  force  of  gravity,  will  fall  just  as  far,  and 
will  be  on  a  level  with  it  at  b.  Both  balls  fall  farther  and 
farther  each  second,  both  being  accelerated  in  the  same  de- 
gree because  it  is  done  by  the  same  force.  The  projected 
ball  will  reach  d  when  the  falling  ball  is  at  c,  and  the  plane 
at  /  when  the  falling  ball  is  at  6,  the  foot  of  the  tower. 


116 


NATURAL   PHILOSOPHY. 


D 


The  same  holds  true  in  all  cases.  A  bullet  dropped  from 
a  level  with  the  barrel  of  a  gun,  paradoxical  as  it  may 
seem,  will  fall  to  the  ground  no  sooner  than  one  which  is 
shot  from  the  gun. 

68.  All  Falling  Bodies  really  Projected.  —  When  a  body 
falls  from  any  height,  it  does  not,  as  you  have  already  seen 
in  §  66,  fall  in  a  straight  line,  as  it  appears  to  do.  It  falls 
in  a  curved  line,  for,  like  all  projectiles,  it  is  acted  upon  by 
a  horizontal  force  as  well  as  the  force  of  gravity.  But  what 
is  this  horizontal  force  ?  It  is  the  motion  which  the  body 
has  in  common  with  the  earth  in  its  rotation  on  its  axis. 
In  this  rotation  the  height  from  which  the  body  falls  goes 
to  the  eastward  1500  feet  in  a  second.  If,  therefore,  the 
body  did  not  partake  of  the  motion  of  the  earth,  and  de- 
scended in  a  straight  line  in  a  second,  it  would  reach  the 
ground  1500  feet  westward  from  the  foot  of  the  height 
whence  it  fell.  But  it  does  partake  of  the  earth's  motion, 
A  -^-44^  o  and,  moving  eastward  as  fast  as  the 
height,  it  describes  the  curved  line  of 
a  projectile.  Suppose  a  ball  falls  from 
Fig.  84.  -0  a  height,  A,  Fig.  84.  and  in  a  second  of 


MOTIONS    OF   MATTER.  117 

time  that  height  passes  to  C.  The  forward  or  projectile 
force  would  tend  to  carry  the  ball  to  C,  and  the  force 
of  gravity  would  tend  to  carry  it  to  B.  But  both  forces 
acting  together,  it  pursues  a  middle  path,  and  this  path 
is  a  curved  line,  because  one  of  the  forces  is  a  continued 
force.  (See  §  63.)  For  the  same  reason,  if  a  ball  be 
dropped  from  a  railway  car  in  motion,  and  it  take  a  sec- 
ond to  fall,  at  the  end  of  that  second  it  will  strike  the 
floor  just  under  that  part  of  the  car  from  which  it  fell. 
Although  the  car  may  have  moved  a  considerable  distance, 
the  dropped  ball,  partaking  of  its  motion,  goes  along  with 
it  in  its  fall.  For  the  same  reason,  a  ball  dropped  from  a 
masthead  when  a  ship  is  in  motion,  partakes  of  the  mo- 
tion of  the  ship.  The  ball  in  each  of  these  cases  describes 
in  its  fall  a  curved  line. 


s. 


G9.  Motion  in  Orbits. — Why  is  it,  let  us  ask,  that  a  cannon-ball 
shot  horizontally  from  some  great  height  will  not  revolve  around  the  earth 
like  the  moon.  It  has  the  same  two  forces  acting  upon  it  as  the  moon  has 
— viz.,  a  projectile  force  and  the  attraction  of  the  earth — and  both  ball 
and  moon  describe  a  curve  in  their  motion.  But  the  curve  of  the  ball 
bends  to  the  earth,  while  that  of  the  moon  ever  sweeps  around  the  earth. 
Why  is  this?  In  the  first  place,  the  resistance  of  the  air  continually  re- 
tards the  velocity  of  the  ball.  But,  secondly,  even  if  the  ball  could  be 
projected  from  an  elevation  sufficiently  high  to  be  outside  of  the  atmos- 
phere, the  force  of  the  projection  would  not  be  great  enough.  We  know, 
from  the  rate  of  progress  of  the  heavenly  bodies  in  their  orbits,  that  it 
would  require  an  immense  velocity  to  keep  the  ball  from  being  brought  to 
the  earth  by  its  attraction.  The  Creator  of  these  worlds,  when  he  launched 
them  into  their  orbits,  gave  them  precisely  that  impulse  which  is  needed 
to  balance  the  centripetal  force  (§  73)  of  attraction,  and  so  they  pursue  a 
middle  course  between  the  two  directions  in  which  these  two  forces  tend 
to  carry  them.  And  as  their  velocities  have  never  been  retarded  by  the 
resistance  of  air  or  any  other  substance,  they  have  been  ever  the  same  from 
the  beginning. 

70.  Newton's  Laws  of  Motion.  —  By  investigating   the 


118  NATURAL   PHILOSOPHY. 

principles  of  motion,  Sir  Isaac  Newton  arrived  at  throe 
laws,  which  have  been  in  some  measure  anticipated  in  the 
preceding  sections.  These  laws  may  be  briefly  stated  as 
follows : 

I.  A  body  free  from  the  interference  of  external  matter 
or  force  will,  if  at  rest,  remain  at  rest,  and  if  in  motion, 
will  move  uniformly  in  a  straight  line. 

II.  A  given  force  always  produces  the  same  effect,  wheth- 
er the  body  on  which  it  acts  be  in  motion  or  at  rest,  and 
whether  it  be  acted  upon  by  one  or  more  forces  simulta- 
neously. 

III.  Action  and  reaction  are  equal  and  opposite. 

These  laws  are  far  more  comprehensive  than  they  at  first 
sight  appear,  and  require  some  explanation. 

The  first  part  of  the  first  law  follows  from  inertia,  §  14; 
a  body  at  rest  will  remain  at  rest  until  some  external  force 
causes  its  motion ;  and  a  body  has  no  power  in  itself  to 
change  the  rate  or  direction  of  its  motion.  Hence  in  the 
communication  of  motion  a  certain  amount  of  time  elapses 
before  its  effects  are  made  evident.  Of  this  we  have  many 
examples,  some  of  which  are  given  in  the  following  sec- 
tions. 

71.  Inertia  Shown  in  the  Communication  of  Motion. — 
When  the  sails  of  a  vessel  are  first  spread  to  the  wind,  the 
vessel  does  not  move  swiftly  at  once,  for  some  time  is  re- 
quired for  the  force  applied  to  overcome  the  inertia  of  so 
large  a  mass,  and  to  put  it  in  rapid  motion.  Horses  make 
a  greater  effort  to  start  a  load  than  they  do  to  keep  it  in 
motion  after  it  is  started.  If  a  person  stand  up  in  a  car- 
riage, and  the  horses  start  off  suddenly,  he  falls  backward, 
because  his  body,  from  its  inertia,  does  not  readily  and  at 
once  partake  of  the  motion  of  the  carriage.  If  a  person 
start  forward  quickly  with  a  waiter,  filled  with  glasses,  in 
his  hands,  the  glasses  will  slide  backward. 


MOTIONS    OP    MATTER.  119 

The  foregoing  illustrations  show  that  it  requires  some  time  to  communi- 
cate motion  to  any  body.  We  .will  give  some  illustrations  of  this  fact  of  a 
more  striking  character.  If  a  ball  be  thrown  against  an  open  door,  it  will 
move  the  whole  door,  and  perhaps  shut  it ;  but  the  same  ball  fired  frorr 
a  rifle  will  pass  through  the  door  without  moving  it  perceptibly.  In  tht 
latter  case  its  velocity  is  so  great  that  there  is  not  time  enough  to  com- 
municate motion  to  the  whole  door,  and  it  moves  only  that  part  of  it  with 
which  it  comes  in  contact.  A  bullet  thrown  with  but  little  force  against  a 
window  will  crack  a  whole  pane  of  glass ;  but  if  shot  from  a  pistol,  it  mere- 
ly makes  a  round  hole.  A  cannon-ball  having  a  great  velocity  may  pass 
through  the  side  of  a  ship,  doing  perhaps  comparatively  little  damage, 
while  one  moving  with  much  less  velocity  may  do  vastly  more  damage  by 
splintering  the  wood  to  a  considerable  extent.  For  the  same  reason  a  rapid 
ball  hitting  a  person  may  occasion  less  suffering  and  do  less  harm  than  a 
slow  ball ;  for  a  rapid  ball  kills  merely  the  parts  which  it  touches,  leaving 
the  flesh  around  in  a  sound  state,  while  the  slow  ball  bruises  over  a  large 
space.  If  a  large  pitcher  filled  with  some  heavy  liquid  be  quickly  taken  up, 
the  handle  will  break,  leaving  the  pitcher  behind.  Large  dishes  are  some- 
times broken  in  this  way  when  heavily  loaded. 

72.  Inertia  Shown  in  the  Disposition  of  Motion  to  Con- 
tinue.— As  in  the  case  of  the  ship,  in  the  first  illustration  in 
§  71,  it  takes  time  to  communicate  motion  to  the  whole 
ship,  or,  in  other  words,  to  overcome  its  inertia,  so,  when 
the  ship  is  once  in  rapid  motion,  it  does  not  stop  suddenly 
when  the  sails  are  taken  down,  but  its  inertia  tending  to 
keep  it  moving  is  gradually  overcome  by  the  resistance  of 
the  water.  If  a  person  stand  up  in  a  carriage  in  motion, 
and  the  horses  suddenly  stop,  he  will  be  thrown  forward, 
for  his  body  has  a  motion  in  common  with  the  carriage, 
and  from  inertia  is  disposed  to  go  on  when  the  carriage 
stops.  When  you  strike  your  foot  against  anything  to  get 
the  snow  off,  you  give  the  foot  and  the  snow  a  common 
motion  together,  then  arresting  the  motion  of  the  foot,  the 
snow  through  inertia  passes  on.  The  same  thing  is  illus- 
trated in 'striking  two  books  together  to  remove  the  dust.  If 
a  ship  strike  upon  a  rock,  everything  on  board  which  is  loose 


120  NATURAL   PHILOSOPHY. 

is  dashed  forward.  The  earth,  as  it  revolves  on  its  axis, 
has  a  velocity  at  the  equator  of  about  1000  miles  an  hour. 
If  this  revolution  should  be  suddenly  arrested,  everything 
loose  on  its  surface,  having  acquired  the  motion  of  the 
earth,  would  be  at  once  thrown  eastward,  just  as  the  fur- 
niture, etc.,  on  board  ship  are  dashed  forward  when  the 
vessel  is  stopped  by  running  against  a  rock.  All  the 
houses,  monuments,  and  structures  of  every  kind  would 
fall  prostrate  eastward. 

An  Equestrian  Feat, — In  the  feat  of  jumping  over  a  cord 
from  the  back  of  a  galloping  horse,  represented  in  Fig.  85, 

the  only  exertion  made 
by  the  rider  is  to  raise 
himself  sufficiently  to 
pass  over  the  cord.  He 
comes  down  again  upon 
the  horse's  back,  simply 
because  of  the  motion 
which  he  has  in  com- 
mon with  the  horse,  his  feet  going  in  the  path  represent- 
ed by  the  dotted  line.  If  he  should  attempt  to  throw 
himself  forward,  as  in  leaping  from  the  ground,  he  would 
go  too  far,  and  perhaps  strike  upon  the  horse's  neck  in- 
stead of  his  back.  Skill  in  jumping  from  a  moving  car- 
riage consists  in  making  the  proper  allowance  for  the 
forward  motion  which  is  had  in  common  with  the  car- 
riage. Most  persons  are  apt  to  overdo  the  matter,  and 
so  come  to  the  ground  prostrate,  and  with  unnecessary 
violence. 

A  Case  in  Court. — A  dashing  young  man  driving  a  light  phaeton  ran 
against  a  heavy  carriage.  His  father  was  induced  by  his  son's  represen- 
tations to  prosecute  the  driver  of  the  carriage  for  driving  too  fast.  A 
knowledge  of  motal  inertia  very  readily  decided  the  case.  The  son  and 
his  servant  both  declared  in  the  witness-stand  that  the  shock  of  the  car- 


MOTIONS    OF    MATTEK.  121 

riage  against  the  phaeton  was  so  great  that  they  were  thrown  over  the 
horses'  heads.  They  thus  proved  themselves  guilty  of  the  fast  driving,  for 
it  was  their  own  rapid  motion  that  threw  them  out  when  the  phaeton  was 
stopped  by  running  against  the  carriage.  The  following  case  is  a  parallel 
one :  If  two  boats — the  one,  of  large  size,  sailing  slowly  up  stream ;  the 
other,  a  small  one,  sailing  rapidly  down — run  against  each  other,  a  man 
standing  in  the  bow  of  the  one  going  down  will  be  thrown  much  farther 
forward  than  one  standing  in  the  bow  of  the  other. 

73.  Centrifugal  Force.  —  The  second  part  of  the  first  law 
states  that  if  a  body  in  motion  be  not  interfered  with,  it  will 
move  uniformly  in  a  straight  line.  This  disposition  of  mo- 
tion to  be  straight  is  well  illustrated  by  a  consideration  of 
centrifugal  force ;  but,  before  explaining  it,  we  will  briefly 
mention  the  nature  of  curved  motion. 

We  have  shown  in  §  60  that  when  two  or  more  forces 
act  upon  a  body  simultaneously,  the  motion  resulting  is  in 
a  straight  line.  If,  however,  you  have  understood  the  ex- 
planation of  the  parabolic  path  of  a  projectile  (§  67),  you 
have  observed  that  two  forces  may  also  produce  curved 
motion,  provided  one  of  them  communicate  a  single  im- 
pulse and  the  other  a  succession  of  impulses. 

Of  this  we  have  a  familiar  example  in  a  ball  whirled 
around  at  the  end  of  a  string.  You  can  give  it  an  impulse, 
and  then,  holding  it  in  your  hand,  let  it  whirl.  Here  the 
impulse  given  the  ball  is  one  force,  and  the  tension  of  the 
string  is  the  other,  the  latter  acting  continuously.  Your 
hand  holding  the  end  of  the  string  is  the  centre  about 
which  the  motion  revolves;  the  impulse. which  you  have 
given  the  ball  tends  to  make  it  fly  away  from  the  centre  in 
a  straight  line,  and  hence  is  called  the  centrifugal  force ; 
the  tension  of  the  string  keeps  it  from  thus  flying  off,  and 
is  called  the  centripetal  force. 

This  will  appear  clearer  by  examining  the  diagram,  Fig. 
86.  If  the  hand  holding  the  string  be  at  A,  the  impulse 
given  the  ball  B  will  tend  to  move  it  in  the  direction  B  C, 


122 


NATURAL   PHILOSOPHY. 


Fig.  86. 


but  the  string  A  B  pulls  it  towards 
the  centre;  and  when  the  ball  reaches 
any  other  point,  as  D,  it  is  prevented 
E  from  pursuing  the  straight  path  D  E 
by  the  same  cause ;  consequently,  the 
ball  revolves  in  a  circle.  That  the 
ball  would  take  the  direction  B  C  but 
for  the  resistance  of  the  string  can 
easily  be  shown  by  experiment :  in  place  of  a  ball  fastened 
to  a  string  we  may  use  a  stone  in  a  sling, 
and  the  instant  the  stone  is  set  at  liberty  it 
will  dart  off  as  straight  as  an  arrow  (Fig.  87). 

When  the  earth,  at  the  creation,  was  put  in  motion,  it 
would  have  moved  in  a  perfectly  straight  line  were  it 
not  constantly  drawn  towards  the  sun  by  attraction,  the 
continuous  action  of  this  latter  force  being  the  same  as 
the  tension  of  the  string  in  the  case  of  the  whirling  ball. 
The  force  of  attraction,  then,  is  the  centripetal  force  of 
the  earth,  and  the  impulse  which  was  given  to  it  by  the 
Creator  in  the  beginning  is  its  centrifugal  force;  and, 
balanced  between  these  two  forces,  the  earth  and  all  the 
heavenly  bodies  move  uniformly  onward  in  their  orbits. 
The  centrifugal  force  in  these  illustrations  is  simply  the 
tendency  of  motion  to  a  straight  line,  which  is  constantly 
counteracted  by  the  centripetal  force. 

•<  74:  Illustrations  of  Centrifugal  Force. — 
When  a  wet  mop  is  whirled,  the  water  flies 
off  in  every  direction  by  its  centrifugal  force. 
On  the  same  principle  a  dog,  coming  out  of 
the  water,  shakes  off  the  water  by  a  semi- 
rotary  motion. — When  a  suspended  bucket 
of  water  is  revolved  swiftly,  the  water  rises 
high  on  its  sides,  and  leaves  a  hollow  in  the 
middle.  It  is  the  tendency  to  fly  away  from 
the  centre  of  motion  that  causes  this.  If  the 


Fig.  87. 


MOTIONS    OF   MATTEK. 


123 


bucket  be  held  firmly  by  the  cord  and  swung 
swiftly  around  the  hand  as  a  centre,  the  cen- 
trifugal force  of  the  water  against  the  bottom 
and  sides  will  prevent  its  escaping,  even  when 
the  bucket  is  upside  down,  as  shown  in  Fig.  88. 
— Large  wheels,  revolving  with  great  velocity, 
have  been  broken  by  the  centrifugal  force  of 
their  particles,  and  hence  the  necessity  of  having 
such  wheels  made  very  strong.  The  immense 
grindstones  used  in  gun-factories  have  some- 
times been  broken,  through  in  the  middle,  or 
have  burst  into  pieces  with  destructive  vio- 
lence from  the  same  cause. — A  man  riding 
horseback  on  turning  a  sharp  corner  inclines 
his  body  towards  the  corner,  to  avoid  being 
thrown  off  by  the  centrifugal  force.  So,  in 
the  feats  of  the  circus,  a  man  standing  on  a 
horse  running  at  full  speed  around  the  ring 
inclines  his  body  strongly  inwards,  as  shown 
in  Fig.  89.  The  horse  also  instinctively  in- 
clines in  the  same  direction  for  the  same  rea- 
son. If  the  rider  find  himself  in  danger  of 
falling,  by  making  the  horse  go 
a  little  faster,  thus  adding  to 
the  centrifugal  force, 

the  difficulty  is  relieved. — The 
centrifugal   force    is    made 
use  of  in  milling.     The 
grain  is  admitted  be- 
tween two  circular 
stones  by  a  hole  in 
the   centre   of  the 
upper  one,  and  as 
Fior.89.  ^ne  stone  revolves 


Fig.  88. 


124  NATURAL   PHILOSOPHY. 

it  constantly  moves  towards  the  circumference,  and  there 
escapes  as  flour. 

Bends  in  Rivers. — We  see  the  operation  of  centrifugal  force  in  the 
bends  of  rivers.  When  a  bend  has  once  commenced  in  a  river,  it  is  apt 
to  increase,  for  as  the  water  sweeps  along  the  outer  bank  of  the  bend  it 
presses  strongly  against  it,  just  as  the  water  in  the  whirled  bucket  presses 
against  its  sides,  by  its  centrifugal  tendency,  or,  in  other  words,  its  ten- 
dency to  assume  a  straight  motion.  Of  course,  the  result  is  a  wearing-awny 

of  this  outer  bank,  and  in  propor- 
tion to  the  looseness  of  the  mate- 
rial of  which  it  is  composed  and 
the  velocity  of  the  river's  current. 
And  when  one  bend  is  formed,  an- 
Fig.  90.  other  is  apt  to  form  below,  but  on 

the  opposite  bank.  The  water,  by  sweeping  along  the  bend  a,  Fig.  90,  is 
directed  by  it  towards  the  opposite  bank  at  b,  and  makes  a  bend  there 
also.  It  is  in  this  way  that  a  river,  running  through  a  loose  soil — 
the  Mississippi,  for  example — acquires  a  very  serpentine  course. 
As  the  water  in  the  whirled  bucket  rises  around  the  sides,  so  in 
the  river  the  water  will  be  higher  against  the  bank  a  than  on  the 
opposite  side.  Eddies  and  whirlpools  are  produced  on  the  same 
principles,  when  water  is  obliged  to  turn  quickly  around  some  pro- 
jecting point.  If  a  current  were  moving  swiftly  along  the  shore 
a  towards  the  point  6,  Fig.  91,  it  would  be  directed  outwards  by 
the  resistance  of  this  projection,  and  so  a  depression  would  be  left 
at  c,  just  behind  it,  and  this  depression  would  be  surrounded  by  a 
revolving  body  of  water.  Fig.  91. 

75.  Application  of  Centrifugal  Force  in  the  Arts. — Much 
use  is  made  of  centrifugal  force  in  the  arts,  and  we  will 
mention  a  few  examples.  In  the  art  of  pottery  the  clay 
is  made  to  revolve  on  a  whirling  table,  the  workman  at 
the  same  time  giving  the  clay  such  shape  as  he  chooses 
with  his  hands  and  various  instruments.  In  doing  this  he 
constantly  pays  attention  to  the  centrifugal  force,  giving 
the  table  a  velocity  proportioned  to  the  amount  of  this 
force  which  is  needed  in  each  stage  of  the  operation.  One 


MOTIONS    OF   MATTER. 


125 


of  the  most  beautiful  applications  of  this  force  is  in  the 
manufacture  of  common  window-glass  as  formerly  conduct- 
ed. The  glass-blower  gathered  up  on  the  end  of  his  iron 
tube  a  quantity  of  the  melted  glass,  and  blew  it  out  into  a 
large  globe.  When  it  was  of  sufficient  size  and  thinness, 
he  placed  it  on  a  rest,  as  shown  in  Fig.  92.  A  second  man 
then  came  with  a  rod  having  some  melted  glass  on  the  end, 
and  attached  this  to  the  globe  at  a  point  opposite  to  that 
where  the  tube  of  the  first  man  joins  it.  Then  a  boy 


gave  this  tube  a  quick  blow  and  severed  its  connection 
with  the  globe,  leaving  a  hole  in  the  globe  where  the  glass 
breaks  out.  The  second  man,  having  the  globe  attached 
to  his  rod,  carried  it  to  a  blazing  furnace,*and,  resting  the 
rod  on  a  bar  at  its  mouth,  put  the  globe  directly  into 
the  flame.  The  glass  being  soon  softened,  he  whirled  the 
globe  continually  around.  The  hole  in  the  globe  enlarged 
by  the  centrifugal  force,  and  at  length  by  this  force  the 
globe  was  changed  into  a  flat,  circular  disk,  from  which 
were  cut  panes  of  glass. 

In  sugar-refineries  the  crystallized  sugar  is  freed  from 

P 


126 


NATURAL   PHILOSOPHY. 


the  viscid  molasses  by  being  placed  in  a  box  revolving  with 
great  speed;  the  liquid  is  thrown  off  by  the  centrifugal 
force,  and  collected  in  a  suitable  manner.  This  method  of 
drying  substances  by  means  of  centrifugal  motion  is  fre- 
quently adopted  in  the  arts ;  perhaps  the  most  curious  ap- 
plication is  to  the  honey-comb.  It  has  been  observed  that 
honey-bees  provided  with  a  clean  comb  will  at  once  proceed 
to  fill  it  with  honey ;  accordingly,  filled  combs  are  carefully 
shaved  to  remove  the  caps  on  the  cells,  and  placed  in  a  cen- 
trifugal machine.  When  the  machine  is  set  in  motion,  the 
honey  is  thrown  out  of  the  cells  quite  perfectly,  and  the 
emptied  comb  is  replaced  in  the  beehive.  By  this  means 
the  valuable  time  of  the  busy  bees  is  economized,  and  they 
are  spared  the  trouble  of  making  fresh  wax  and  new  combs. 
Steam- Governor. — The  operation  of  centrifugal  force  is 
beautifully  exemplified  in  this  regulator  of  the  steam-en- 
gine. It  consists  of  two 
heavy  balls,  Fig.  93,  sus- 
pended by  bars  from  a 
vertical  axis,  the  bars 
being  connected  to  the 
axis  by  hinges.  The  bars 
have  also  a  hinged  con- 
nection at  their  lower 
ends  with  two  smaller 
bars,  and  these  latter 
have  a  similar  connec- 
tion with  a  collar  that 
slides  up  and  down  on 

the  axis.    Now  the  fast- 

Fig- 93>  er  the  axis  turns,  the  far- 

ther the  balls  fly  out  from  it,  from  the  centrifugal  force, 
and  the  higher  the  collar  slides  up  on  the  axis.  From  the 
collar  extends  a  lever.  This  is  connected  with  a  valve  in 


MOTIONS    OF    MATTER.  127 

the  steam-pipe,  and  so  regulates  the  amount  of  steam  that 
enters  the  working  part  of  the  engine.  The  object  of  this 
ingenious  contrivance  is  to  make  the  engine  regulate  its 
own  velocity.  When  it  is  not  working  too  fast,  the  valve 
in  the  steam-pipe  is  wide  open.  But  the  moment  that  the 
engine  begins  to  move  too  rapidly,  the  balls  extend  out  far 
from  the  axis,  so  that  the  collar  rises,  and  by  the  lever 
partly  closes  the  valve.  Less  steam,  therefore,  can  come 
to  the  engine  ;  and  the  engine  working,  in  consequence,  less 
rapidly,  the  balls  fall  again,  opening  the  valve.  You  see, 
then,  that  the  regulation  of  this  valve  by  the  governor 
effectually  prevents  the  engine  from  running  at  a  danger- 
ously high  speed. 

76.  Shape  of  the  Earth  Influenced  by  Centrifugal  Force. — 
If  the  potter  should  make  a  ball  of  soft  clay  revolve  rap- 
idly around  on  a  stick  run  through  it,  the  ball  would  bulge 
out  at  the  middle,  where  the  centrifugal  force  is  greatest, 
and  would  be  flattened  at  the  ends  where  the  stick  runs 
through  it.  This  is  precisely  what  has  happened  to  the 
earth.  At  the  equator,  where  the  centrifugal  force  is  great- 
est, it  has  bulged  out  about  thir- 
teen miles,  while  it  is  -flattened 
at  the  poles.  This  shape  was  of 
course  assumed  before  the  earth 
became  solid.  Fig.  94  represents  E 
the  shape  of  the  earth,  N  S  being 
the  polar  diameter,  and  E  E'  the 
equatorial  diameter.  The  depres- 
sion at  the  poles  is  much  exag- 
gerated in  the  figure  in  order  to 
make  the  shape  manifest.  The  tendency  to  take  this  shape 
from  the  centrifugal  force  may  be  illustrated  by  the  in- 
strument represented  in  Fig.  95.  It  consists  of  two  circu- 
lar hoops  of  brass  connected  with  an  axis,  b  a.  The  hoops 


128 


NATUKAL  PHILOSOPHY. 


Fig.  95. 


are  fastened  to  the  axis  at  «, 
but  are  left  free  at  b.  By 
some  simple  machinery  at  the 
top  the  hoops  can  be  made  to 
revolve  rapidly;  and  bulging 
out  at  the  sides  by  the  centrif- 
ugal force,  they  slide  down  on 
the  axis  at  b. 

/  7Y.  Uniformity  of  Motion. — 
A  third  point  in  the  first  law 
refers  to  the  uniformity  of  mo- 
tion in  the  absence  of  any  in- 
terfering cause.  This  uniformity  is  true  both  of  the  direc- 
tion and  of  the  velocity. 

Suppose  a  body  to  be  set  in  motion,  and  to  meet  with 
no  opposition  from  friction,  or  the  resistance  of  air,  or  at- 
traction, it  would  move  on  forever,  and  with  the  same  ve- 
locity with  which  it  began.  Now  precisely  these  circum- 
stances exist  in  the  motion  of  the  heavenly  bodies  in 
their  orbits.  They  are,  it  is  true,  under  the  influence  of 
attraction,  but  in  such  a  way,  as  you  will  soon  see,  as  not 
to  interfere  with  the  uniformity  of  their  motion.  Were  it 
not  for  this  uniformity,  we  should  have  no  regularity  of 
times  and  seasons.  It  is  only  by  the  uniform  motion  of  the 
earth  round  the  sun,  and  round  its  own  axis,  that  we  can 
calculate  for  to-morrow,  or  next  week,  or  next  year.  If 
these  motions  were  irregular,  it  would  throw  confusion  into 
all  our  calculations  for  the  future  and  all  our  recollections 
of  the  past.  We  can  measure  time  by  nothing  else  than  reg- 
ular motion  ;  and  were  there  no  regular  motion,  we  should 
have  merely  the  very  inaccurate  measure  furnished  by  our 
sensations.  To  measure  time  with  accuracy,  we  take  some 
great  and  extensive  uniform  motion  as  our  standard.  Thus, 
the  revolution  of  the  earth  around  the  sun  we  take  as  one 


MOTIONS    OF   MATTER.  129 

division  of  time,  and  call  it  a  year.  We  observe  that  dur- 
ing this  time  it  whirls  around  on  its  own  axis  365  times, 
and  the  time  occupied  by  each  of  these  revolutions  we  call 
a  day. 

The  impossibility  of  producing  on  the  earth's  surface  a  condition  of 
things  similar  to  that  in  the  empty  space  through  which  the  heavenly  bodies 
move  is  an  argument  against  the  attainment  of  perpetual  motion.  It  is 
evidently  impossible  to  annihilate  external  forces,  such  as  gravity,  the  re- 
sulting friction,  etc.,  and  consequently  the  motion  of  any  object  will  not  be 
uniform,  but  continually  retarded.  Perpetual  motion  (the  dream  of  vision- 
ary philosophers  of  many  centuries)  is,  then,  a  mechanical  impossibility. 

78.  The  Second  Law  of  Motion. — The  second  la\v  of  mo- 
tion states  that  a  given  force  always  produces  the  same 
effect,  whether  the  body  on  which  it  acts  be  in  motion  or 
at  rest,  and  whether  it  be  acted  upon  by  one  or  more  forces 
simultaneously.     This  is  in  reality  an  expansion  of  the  first 
law,  and  its  principles  have  been  anticipated  in  speaking 
of  compound  motions  (§  60)  and  of  projectiles  (§  67).    Thus, 
whatever  the  number  of  forces  acting  upon  a  body,  each 
force  may  be  regarded  as  producing  independently  its  own 
change  of  motion.      When  a  ball  is  thrown  horizontally, 
the  deviation  from  a  straight  line  leads  to  the  inference 
that  it  is  affected  by  another  and  vertical  force — that  of 
gravity. 

79.  The  Third  Law  of  Motion. — The  third  law  of  motion 
states  that  action  and  reaction  are  equal — that  is  to  say, 
when  any  of  the  causes  of  motion  act,  the"  action  is  met  by 
an  opposite  and  equal  reaction.     If,  for  example,  a  blow 
be  given,  an  equal  blow  is  received  in  return.     For  this 
reason,  if  one  in  running  hit  his  head  against  the  head  of 
another,  both  are  equally  hurt.     When  a  child  knocks  his 
head  against  a  table,  there  is  sound  philosophy  in  the  com- 
mon saying  that  he  has  given  the  table  as  good  a  blow  as 
he  has  received,  though  it  may  afford  him  no  comfort. 


130  NATURAL   PHILOSOPHY. 

Many  very  interesting  illustrations  of  this  law  of  motion 
suggest  themselves,  of  which  we  will  give  a  few. 

A  swimmer,  pressing  the  water  downward  and  backward 
with  his  hands  and  feet,  is  carried  along  forward  and  up- 
ward by  the  reaction  of  the  water.  And  in  this  case,  as  in 
every  other,  the  greater  the  action,  the  greater  is  the  reac- 
tion ;  in  other  words,  the  more  strongly  he  presses  with  his 
hands  and  feet,  the  more  rapidly  is  he  borne  along  by  the 
reaction  of  the  water  against  the  pressure.  A  boat  ad- 
vances in  proportion  to  the  force  with  which  the  oars  press 
against  the  water.  So  the  rapidity  of  a  steamboat  depends 
on  the  force  with  which  the  paddles  drive  the  water  astern. 
Birds  rise  in  the  air  by  the  reaction  of  the  air  against  their 
wings  as  they  are  pressed  downward.  A  sky-rocket  pur- 
sues its  rapid  flight  because  a  large  quantity  of  gaseous 
matter  issues  from  its  lower  end,  and,  being  resisted  by  the 
air,  its  pressure  throws  the  rocket  upward.  If  a  ship  fire 
guns  from  the  stern,  its  advance  will  be  accelerated ;  but  if 
from  the  bow,  it  will  be  retarded.  When  a  broadside  is 
fired,  the  ship  inclines  to  the  other  side. 

Further  Illustrations.  —  If  a  spring  be  compressed  between  two  equal 
bodies,  it  will  throw  them  off  with  equal  velocities.  If  they  are  un- 
equal, the  velocity  of  the  smaller  body  will  be  greater  than  that  of  the 
larger,  and  in  proportion  to  its  smallness.  For  this  reason,  when  a  ball 
issues  from  a  cannon,  though  the  cannon  and  the  ball  are  equally  acted 
upon  by  the  elastic  or  expansive  force  of  the  gases  set  free  by  burning  the 
powder,  the  gun  is  moved  but  very  little,  because  the  force  is  diffused 
through  so  large  a  mass ;  while  the  ball,  being  so  much  smaller,  moves  with 
great  velocity.  When  a  volcano  throws  stones  from  its  crater,  the  earth 
may  be  compared  to  the  cannon,  the  stones  to  the  ball,  and  the  explosive 
materials  throwing  the  stones  to  the  exploding  powder  projecting  the 
ball.  Since  the  cannon  is  moved  as  much  as  the  ball,  the  earth  also  is 
moved  as  much  as  the  stones,  the  only  reason  that  it  does  not  move  so 
far  and  so  rapidly  being  that  the  force  is  diffused  through  so  large  a 
bulk.  These  examples  illustrate  very  well  the  relation  of  action  and  re- 
action ;  for  whenever  there  is  an  action  of  one  body  upon  another,  it  is  as  if 


MOTIONS    OF   MATTER. 


131 


a  spring  were  between  the  two  bodies,  acting  equally  upon  both.  When 
a  man  jumps  from  the  ground,  it  is  as  if  a  spring  were  compressed  between 
him  and  the  earth,  and  this  expanding  moves  the  earth  exactly  as  much  as 
it  does  the  man.  He  really  kicks  the  earth  away  from  him.  The  motion 
of  the  earth  is  not  obvious  because  it  is  diffused  through  so  large  a  mass. 
The  case  is  parallel  to  that  of  the  ball  and  cannon.  The  same  force  is 
exerted  upon  the  man  and  the  earth  ;  but  the  man,  like  the  ball,  moves  the 
most,  and  in  proportion  to  his  small  size.  So  when  a  bird  hops  from,  the 
ground,  the  earth  moves  as  really  as  the  bird.  If  the  bird  hop  from  a  twig,' 
you  perceive  that  the  twig  is  moved  by  the  pressing-down  of  the  bird  as  it 
rises.  When  it  starts  from  the  ground,  it  exerts  the  same  downward 
pressure,  and  moves  the  earth  as  really  as  in  the  other  case  it  did  the  twig. 
Of  course,  many  of  these  motions  are  far  too  small  to  be  perceptible,  and 
too  insignificant  to  take  into  account  under  ordinary  circumstances. 

80.  Communication  of  Motion  in  Elastic  Bodies.  —  Addi- 
tional proof  of  the  truth  of  this  law  is  shown  in  the 
communication  of  motion  in  elastic  bodies.  Momentum 
is  transferred  from  one  body  to  another  very  differently 
in  elastic  and  non-elastic  bodies.  As  shown  in  §  61,  when 
one  non- elastic  body  strikes  upon  another,  the  momen- 
tum is  divided  between  them,  and  both  move  on  to- 
gether. Now,  if  a  and  b,  Fig.  96, 
were  elastic  bodies,  as  ivory  balls, 
and  b  should  be  let  fall  against  a, 
it  would  give  all  its  momentum  to 
a.  Therefore  b  would  stop,  and  a 
would  move  on  to  the  same  height 

O 

from  which  b  came.  The  reason  is, 
that  the  velocity  lost  by  b  and  re- 
ceived by  a  is  just  double  what  it 
would  be  if  the  balls  were  non -elastic, 
reason,  if  a  and  #,  being  elastic,  meet  each  other  from 
equal  heights  on  the  arc,  they  will  both  rebound,  and  re- 
turn to  the  same  heights  from  which  they  came.  But  if 
non-elastic,  they  simply  destroy  each  other's  momentum 
and  stop.  The  effect  produced  in  the  former  case  is  just 


0 

Fig.  96. 

For  the  same 


132 


NATURAL   PHILOSOPHY. 


twice  as  great  as  in  the  latter,  as  you  may  see  by  reckon- 
ing on  the  arc.     For  the  same  reason,  too,  if  you  have  a 

row  of  elastic  balls,  as  in  Fig.  97, 
and  let  a  fall  from  the  point  i 
upon  by  it  will  stop  there ;  and 

O  communicating  all  its  momentum 
to  b,  this  momentum  will  pass 
from  b  to  c,  and  so  on  through 
all  the  row  of  balls  to  e,  the  last 
one,  which  will  fly  off  to  the  point  A,  at  the  same  height 
with  i,  the  point  from  which  a  fell.  If  b  be  held  still,  and 
a  be  let  fall  upon  it,  a  will  rebound  to  the  height  from 
which  it  fell,  for  then  the  compressed  elastic  spring  of  each 
ball,  b  being  immovable,  communicates  all  the  motion  to  a. 
It  is  for  this  reason  that  an  elastic  ball,  on  being  thrown 
against  anything  fixed,  rebounds.  If  it  be  thrown  against 
a  perfectly  elastic  body,  it  rebounds  with  a  force  equal  to 
that  with  which  it  is  thrown.  The  transmission  of  sound 
by  the  air  takes  place  in  a  somewhat  similar  manner,  as 
•will  be  shown  in  Chapter  XIV. 


QUESTIONS. 

06.  What  is  said  of  the  course  of  bodies  thrown  into  the  air  ?  Give  the 
comparison  of  a  man  on  a  steamboat.  Why  does  the  atmosphere  move 
with  the  earth? — 67.  Explain  the  parabolic  path  of  projectiles.  What 
three  forces  act  upon  a  stone  thrown  into  the  air  ?  What  is  the  path  of  a 
body  projected  horizontally?  Show  that  the  shape  of  the  curve  is  modified 
by  the  force  exerted.  Show  why  a  ball  dropped  from  the  mouth  of  a  can- 
non will  fall  to  the  ground  in  the  same  time  as  one  fired  from  it. — 68.  By 
what  two  forces  is  a  falling  body  acted  upon  ?  Explain  Fig.  84.  What  is 
the  course  of  a  ball  dropped  from  a  railway  car  or  from  a  mast-head  ?— 60. 
Give  the  comparison  between  the  cannon-ball  and  the  moon.  What  is  said 
of  the  velocities  of  the  heavenly  bodies  ? — 70.  What  are  Newton's  laws  of 
motion?  From  what  principle  does  the  first  law  follow? — 71.  Illustrate 
the  fact  that  inertia  is  shown  in  the  communication  of  motion. — 72.  Give 


THE    SIMPLE   MACHINES.  133 

illustrations  of  inertia  as  shown  in  the  disposition  of  motion  to  continue. 
Describe  and  explain  the  equestrian  feat  mentioned.  What  is  said  of  skill 
in  jumping  from  a  moving  carriage  ?  Relate  the  case  in  court. — 73.  What 
is  stated  in  the  second  part  of  Newton's  first  law  ?  What  is  said  of  curved 
motion  ?  Explain  the  diagram  (Fig.  86).  What  are  centrifugal  and  cen- 
tripetal forces  ?  What  forces  correspond  to  these  in  the  revolution  of  the 
earth  around  the  sun  ? — 74.  Give  illustrations  of  the  various  operations  of 
centrifugal  force.  What  is  said  of  the  formation  of  bends  in  rivers  ?  Show 
how  eddies  and  whirlpools  are  formed. — 75.  How  is  centrifugal  force  used 
in  the  art  of  pottery.  How  in  making  window-glass  ?  How  in  sugar- 
refineries  ?  WThat  is  said  of  the  honey-comb  ?  Describe  and  explain  the 
operation  of  the  steam-governor. — 7G.  What  is  said  of  the  agency  of  cen- 
trifugal force  in  shaping  the  earth?  Explain  the  operation  of  the  ap- 
paratus mentioned. — 77.  What  is  said  of  uniformity  of  motion  ?  What  of 
its  uniformity  in  velocity  ?  State  by  what  means  we  calculate  time.  What 
is  said  of  perpetual  motion  ? — 78.  What  is  stated  in  the  second  law  of 
motion  ? — 79.  What  is  stated  in  the  third  law  of  motion  ?  Illustrate  the 
law.  Give  the  illustration  of  the  spring.  Of  the  cannon-ball.  Of  the 
volcano.  What  is  said  of  the  jumping  of  a  man  from  the  ground  ?  What 
of  the  reaction  in  the  case  of  a  hopping  bird  ? — 80.  Explain  the  additional 
proof  of  the  law  shown  in  the  communication  of  motion  in  elastic  bodies. 


CHAPTER  IX. 

THE    SIMPLE    MACHINES. 

81.  Machines  not  Sources  of  Power.— As  shown  in  §  56, 
the  forces  of  nature  at  our  command  are  few  in  number, 
and  under  many  circumstances  only  one — gravitation — is» 
available.  Nature  seldom  provides  forces  in  a  form  direct- 
ly suited  to  the  accomplishment  of  work,  and  we  therefore 
resort  to  contrivances  called  machines,  by  which  one  form 
or  degree  offeree  is  transformed  into  another,  and  rendered 
serviceable.  From  an  erroneous  idea  of  the  principles  in- 
volved, six  simple  machines,  named  respectively  the  Lever, 
the  Wheel  and  Axle,  the  Inclined  Plane,  the  Wedge,  the 

F2 


134  NA.TUKAL  PHILOSOPHY. 

« 

Screw,  and  the  Pulley,  were  formerly  spoken  of  as  the 
Mechanical  Powers.  These  machines,  however,  are  in  no 
sense  powers,  but  merely  means  of  applying  power  to 
advantage,  and  are  not  in  themselves  sources  of  power. 
No  instrument  or  machine  can  create  power,  and  the  only 
use  of  all  the  variety  of  tools  and  machinery  is  to  enable  us 
to  apply  power  in  such  a  manner,  with  such  a  velocity,  and 
in  such  a  direction  as  to  effect  the  objects  which  we  have 
in  view.  Excepting  old  usage,  there  is  no  reason  why  the 
term  mechanical  powers  should  have  been  confined  to  the 
six  contrivances  above  named.  Any  arrangement  of  solid 
or  rigid  parts,  moving  with  different  velocities  whereby 
one  manifestation  offeree  is  converted  into  another,  equal- 
ly deserves  the  appellation  mechanical  power. 

Before  proceeding  to  a  consideration  of  the  six  simple 
machines,  we  will  explain  a  few  of  the  terms  employed. 
Power  is  the  force  by  which  a  machine  or  instrument  is 
moved.  Weight  is  the  resistance  to  be  overcome.  If  the 
resistance  be  in  some  other  form  than  that  of  weight,  it  is 

TO          i 

called  technically  by  this  name.  So  what  is  called  power 
may  be  in  the  form  of  weight.  The  fulcrum  is  the  point 
on  which  the  instrument  or  machine  is  supported  while  it 
is  in  motion.  * 

82.  Lever  of  the  First  Kind. — A  beam  or  rod  of  wood, 
iron,  or  any  other  material,  resting  at  one  part  on  a  prop, 
or  fulcrum,  about  which  the  beam  can  move  is  called  a 
lever,  the  name  being  derived  from  a  Latin  word  mean- 
ing to  lift.  The  lever  is  the  most  simple  of  all  simple 
machines,  and  is  therefore  in  universal  use.  Though  the 
savage  makes  use  of  few  tools  in  comparison  with  the 
civilized  man,  he  uses  the  lever  almost  constantly  in  some 
form.  The  wedge  is  the  only  one  of  the  other  simple  ma- 
chines that  he  uses  to  any  great  extent.  Levers  are  of 
three  kinds,  commonly  called  the  first,  second,  and  third 


THE    SIMPLE    MACHINES. 


135 


kinds,  the  difference  depending  on  the  relative  position  of 
the  power,  the  weight,  and  the  fulcrum. 

In  the  lever  of  the  first  kind  the  fulcrum,  or  prop,  is  be- 
tween the  weight  and  the  power.  The  common  crow-bar, 
or  hand-spike,  is  a  fa- 
miliar example,  as  seen 
in  Fig.  98 — the  stone, 
S,  or  other  heavy  body 
to  be  moved  being  the 
weight,  the  stone  or 
block  of  wood,  F,  on 
which  the  bar  rests 
being  the  fulcrum,  and 

the  pressure  of  the  hand,  H,  the  power.     The  nearer  the 

fulcrum  to  the  weight,  or  the 
farther  the  power  from  the 
fulcrum,  the  greater  is  the 
force  of  the  lever.  This  may 
be  illustrated  in  Fig.  99.  Here 
the  short  arm  of  the  lever,  as 
it  is  called,  C  W,  is  one  eighth 
of  the  length  of  the  long  arm,  A  C.  If  the  weight  hanging 
at  the  end  of  the  short  arm  be  72  pounds,  a  weight  of  9 
pounds,  or  the  force  of  a  hand  equivalent  to  this,  will  bal- 
ance it  at  the  end  of  the  long  arm.  But  if  the  power 
should  be  applied  at  only  four  times  the  distance  from  the 
fulcrum  at  which  the  weight  is,  then.it  would  require  a 
force  of  18  pounds  to  balance  the  72  pounds  on  the  short 
arm.  Similar  variations  can  be  made  by  altering  the 
length  of  the  short  arm.  The  power  and  the  weight  bal- 
ance each  other  when  the  weight  multiplied  by  the  length 
of  the  short  arm,  and  the  power  multiplied  by  the  length 
of  the  long  arm,  give  equal  products. 

Steelyards  and  the  Balance. — Examples  of  levers  of  this 


Fig.  99. 


136 


NATUEAL  PHILOSOPHY. 


Fig. 100. 


kind  are  very  common. 
In  an  ordinary  balance, 
Fig.  100,  we  have  a  lever 
the  two  arms  of  which 
are  equal,  and  therefore 
equal  weights  suspended 
at  the  ends  balance.  If 
they  be  not  exactly  equal, 
a  heavier  weight  will  be 
necessary  on  the  shorter 
arm.  The  inequality  will 
injure  the  buyer  if  the 
prop  be  too  near  the  pan 
in  which  the  weights  are 
placed,  and  the  seller  if  it  be  too  near  that  which  holds  the 
article  to  be  sold.  Any  difference  can  be  easily  detected 
by  changing  the  places  of  the  article  and  the  weights. 
Whenever  cheating  is  practised  by  the  "  false  balance,"  it 
is  of  course  done  in  a  small  way,  to  avoid  any  observation 
by  the  eye  of  the  inequality  of  the  two  arms  of  the  scale- 
beam,  and  the  weight  of  the  pan  hanging  from  the  shorter 
arm  is  made  a  little  greater  than  that  of  the  other,  so  that 
they  may  balance.  Balances  may  be  rendered  very  accu- 
rate by  making  the  fulcrum  or  pivot  of  hardened  steel,  and 
of  a  wedge  shape,  with  a  sharp  edge,  in  order  to  avoid  fric- 
tion as  much  as  possible. 

The  steelyard,  Fig.  101,  differs  from  the  balance-beam  in 
having  the  arms  of  different  lengths.  The  principles  on 
which  this  instrument  is  constructed  were  developed  in  ex- 
plaining Fig.  99.  With  either  the  balance  or  the  steelyard, 
when  two  weights  balance  each  other,  the  centre  of  the 
weights  and  the  apparatus  taken  together  is  just  over  the 
fulcrum.  Hence  the  necessity  for  placing  the  prop  near  the 
large  weight  when  we  wish  to  balance  it  by  a  small  one. 


THE   SIMPLE    MACHINES. 


137 


Fig.  101. 


Iii  Fig.  101  C  is  the  ful- 
crum, P  the  weight  to  be 
determined,  and  Q  the 
power  applied  in  the  form 
of  a  hanging  weight. 

Other  Examples. — Scissors 
are  double  levers  of  the  first 
kind.  The  fulcrum  is  the  rivet, 
the  weight  or  the  resistance  to 
be  overcome  is  the  article  to  be 
cut,  and  the  power  is  applied  to 
the  long  arms  of  the  levers  by 
the  fingers.  With  large  shears  hard  substances  can  be  cut.  Even  plates 
of  iron  are  cut  like  paper  by  shears  worked  by  a  steam-engine. — Pincers 
are  double  levers.  The  hinge,  or  rivet,  is  the  fulcrum. — The  common 
hammer,  as  used  in  drawing  nails,  is  a  good  example  of  the  power  of  this 
kind  of  lever.  Though  crooked,  it  acts  in  the  same  way  with  a  straight 
lever.  The  fulcrum  is  the  point  on  the  board  where  the  hammer  rests, 
and  this  is  between  the  resistance  to  be  moved,  the  nail,  and  the  power, 
that  is,  the  hand  which  grasps  the  handle. 

83.  No  Gain  of  Power  in  this  Lever. — We  will  now  illus- 
trate the  truth  that  there  is  no  gain  or  saving  of  power  in 
this  lever,  as  might  at  first  thought  seem  to  be  the  case. 
Let  a  b,  Fig.  102,  represent  a  lever, 
and  e  its  fulcrum.  Let  the  arm  a  e 
be  twice  as  long  as  e  b.  A  pound, 
then,  suspended  from  a  will  balance 
two  pounds  at  b.>  If  now,  when  the 
weights  are  suspended,  the  long 
arm  be  raised  so  that  the  lever  oc- 
cupy the  position  represented  by 
the  line  c  d,  and  then  let  go,  the 
one  pound  at  c,  balancing  the  two 


Fig.  102. 


pounds  at  c7,  will  bring  the  lever  again  to  the  position  a  b. 
It  will  be  perceived  that  the  end  of  the  long  arm  of  the 


138 


NATURAL  PHILOSOPHY. 


lever  moves  through  the  space  a  c,  which  is  larger  than 
b  d,  through  which  the  end  of  the  short  arm  moves,  in  the 
same  time.  The  one-pound  weight,  in  fact,  falls  two  feet 
in  raising  the  two-pound  weight  one  foot,  and  it  moves 
twice  as  far  as  a  one-pound  weight  suspended  at  i  would 
do.  If  a  one-pound  weight  could  raise  a  two-pound  weight 
without  thus  moving  through  twice  as  much  space,  an 
actual  gain  of  power  in  the  lever  would  indeed  result.  But 
it  evidently  makes  no  difference  whether  one  pound  moves 
through  two  feet  or  two  pounds  through  one  foot ;  the 
force  is  the  same  in  both  cases.  For  the  momentum  or 
force  of  a  moving  body  is  in  proportion  to  its  weight  and 
velocity  (§  61) ;  and  therefore  the  pound  weight  moving 
through  two  feet  has  as  much  momentum  as  the  two-pound 
weight  moving  through  one  foot  in  the  same  time.  The 
small  weight  does  the  same  amount  of  work  that  the  larger 
one  would  by  moving  twice  as  far  in  the  same  time,  just 
as  a  boy  who  carries  a  load  half  as  large  as  a  man  will 
do  as  much  work  as  the  man  if  he  carry  it  twice  as  fast. 

The  Seesaw. — The  same 
thing  is  illustrated  in  the  see- 
saw, Fig.  103.  The  man,  being 
much  heavier  than  the  boy,  is 
nearer  the  prop  ;  and  as  they 
move  up  and  down  the  boy 
passes  over  a  much  larger  space 
than  the  man,  describing  an  arc 
in  a  much  larger  circle. 

Archimedes's  Lever.  —  Archi- 
medes, a  distinguished  mathema- 
tician and  philosopher  who  lived 
about  250  years  before  the  Chris- 
tian era,  said  that  if  he  could 
have  a  lever  long  enough  and  a 
prop  strong  enough,  he  could  move  the  world  by  his  own  weight.  But  he 
did  not  think  how  fur  he  himself  would  have  to  move  to  do  this,  owing  to 


THE    SIMPLE    MACHINES.  139 

the  vast  difference  between  the  weight  of  his  body  and  that  of  the  earth. 
"He  would  have  required,"  says  Dr.  Arnott,  "to  move  with  the  velocity 
of  a  cannon-ball  for  millions  of  years  to  alter  the  position  of  the  earth  by  a 
small  part  of  an  inch."  Somewhat  analogous  to  this  is  the  case  of  the 
Hydrostatic  Bellows  and  of  Bramah's  Press,  as  will  be  explained  in  Chap- 
ter X.  In  all  these  cases  great  effects  are  produced  by  small  power,  which 
itself  has  to  accomplish  extensive  motion. 

84.  Lever  of  the  Second  Kind. — In  the  second  kind  of 
lever  the  weight  is  between 
the  fulcrum  and  the  power,  as 
shown  in  Fig.  104.  The  same 
rule  of  equilibrium  applies  here 
as  in  the  case  of  the  lever  of  the 
first  kind.  The  72  pounds  of 
weight  can  be  sustained  by  8 
pounds  of  power,  because  the  Fig.  104. 

power  acts  on  the  lever  at  9  times  the  distance  from  the 
fulcrum  that  the  weight  does,  for  1  x  72  =  9x8.  The  com- 
mon wheelbarrow,  Fig.  105, 
is  an  example  of  this  kind 
of  lever.  The  point  at 
which  the  wheel  presses 
on  the  ground  is  the  ful- 
crum, and  the  weight  is 
the  load,  its  downward 
pressure  from  its  centre 
- 105.  of  gravity  being  indicated 

at  M.  Of  course,  the  nearer  the  load  is  to  the  fulcrum,  the 
easier  it  is,  on  starting,  to  raise  the  handles.  The  common 
hand-barrow  furnishes  another  illustration  (see  Fig.  106). 
If  the  load  be  placed  in  the  centre,  each  of  the  men  carries 
half,  for  the  pole  becomes  a  lever,  of  which  each  porter 
is  a  fulcrum  as  regards  the  other;  if  the  load  be  shifted 
towards  one  of  the  men,  he  will  have  to  carry  a  larger 


140 


NATURAL   PHILOSOPHY. 


share  than  the  other. 
The  crow-bar  can  be 
used  as  a  lever  of  the 
second  kind  when  its 
point  is  placed  be- 
yond the  weight  to 
be  raised.  The  chip- 
ping-knife,  Fig.  107, 
Fig.  loo.  is  another  example. 

The  end  attached  to  the  board  is  the  fulcrum,  the  press- 
ure on  the  handle  the 
power, -and  the  resistance 
of  the  substance  to  be 
cut  is  the  weight.  Nut- 
crackers operate  in  a  sim- 
ilar manner.  In  shutting 
a  door,  by  pushing  it 
near  its  edge  we  move 

a  lever  of  this  kind.     The  hinge  is  the  fulcrum,  and  the 

weight  is  between  this 
and  the  hand.  We  see, 
then,  the  reason  that  the 
slight  push  of  a  hand 
shutting  the  door  may 
even  crush  a  finger  when 

Fig.  108. 

where  the  hinges  are. 
the  fulcrum  that  the  power  moving  through  a  great  space 
acts  upon  it  with  immense  force.  The  same  explanation 
applies  to  the  severe  bite  of  the  finger  when  it  is  caught  in 
the  hinge  of  a  pair  of  tongs.  The  oar  of  a  boat  is  a  lever 
of  this  kind,  the  weight  to  be  moved  being  the  boat,  and 
the  fulcrum,  singularly  enough,  being  the  unstable  water. 
85.  Lever  of  the  Third  Kind. — In  the  third  kind  of  lever 


caught  in  it  at  the  side 
The  finger  is  a  resistance  so  near 


THE    SIMPLE   MACHINES.  141 

the  power  is  between  the  fulcrum 
and  the  weight,  as  seen  in  Fig. 
109.  In  the  first  two  kinds  of 
lever  the  power  may  be  less  than 
the  weight,  but  in  this  the  power 
must  always  be  greater  than  the 
weight.  The  advantage  of  this 
lever  consists  in  the  great  extent 

of  motion  obtained.  Applying  the  same  rule  here  as  in  the 
other  levers,  let  us  calculate  the  result.  If  the  weight,  as 
in  Fig.  109,  be  9  times  as  far  from  the  fulcrum  as  the  pow- 
er, it  will  require  a  power  equal  to  a  weight  of  648  pounds 
to  sustain  a  weight  of  72  pounds,  for  9  x  V2  =  l  x  648. 

Examples. — When  a  man  puts  his  foot  against  the  end  of  a  ladder,  and 
raises  it  by  taking  hold  of  one  of  the  rounds,  the  ladder  is  a  lever  of  this 
kind.  It  is  evident  that  he  spends  his  force  upon  it  at  a  great  mechanical 
disadvantage,  for  the  power  is  applied  much  nearer  to  the  fulcrum  than  is 
the  weight  of  the  ladder,  taken  as  a  whole.  If  you  push  a  door  to  by  plac- 
ing your  hand  very  near  the  hinges,  you  do  not  shut  it  as  easily  as  when 
you  take  hold  of  it  at  its  edge.  In  the  first  case  it  is  a  lever  of  the  third 
kind,  and  the  hand  moves  through  a  small  space,  and  therefore  must  exert 
a  considerable  force ;  while  in  the  latter  case  the  door  is  a  lever  of  the  sec- 
ond kind,  and  the  hand,  moving  through  a  greater  space,  puts  forth  less 
force.  When  we  use  a  pair  of  tongs,  we  use  a  pair  of  levers  of  the  third 
kind.  They  are  an  instrument  in  which  convenience  rather  than  power  is 
needed.  We  cannot  grasp  anything  very  firmly  with  them  because  the 
power  is  so  much  nearer  to  the  fulcrum  than  the  weight  to  be  lifted.  For 
this  reason,  a  pinch  with  the  ends  of  the  tongs  is  of  small  moment  com- 
pared with  one  in  the  hinge.  The  anatomical  structure  of  animals  fur- 
nishes a  most  beautiful  example  of  this  lever.  Take,  for  example,  the 
principal  muscle  which  bends  the  elbow,  as  represented  in  Fig.  110.  This 
comes  down  from  the  shoulder  in  front  of  the  bone  of  the  arm,  and  is  in- 
serted just  below  the  elbow-joint  into  one  of  the  bones  of  the  forearm.  It 
pulls  upon  the  forearm  very  near  the  fulcrum,  which  is  the  elbow-joint,  and 
so  acts  at  a  great  mechanical  disadvantage.  The  object  of  this  arrange- 
ment is  to  secure  quickness  of  movement,  which  is  here,  as  in  almost  all 
muscular  motions,  of  more  importance  than  great  strength.  When  great 


142 


NATURAL   PHILOSOPHY. 


110. 

weights  are  lifted,  the  fact  that  the  muscles  act  at  such  mechanical  disad- 
vantage makes  the  exhibition  of  power  wonderful. 

86.  Compound  Levers.  —  When  several  levers  are  con- 
nected together,  we  call  the  whole  apparatus  a  compound 

lever.     Let  each  of 


c 


Fig.  111. 


the  levers  in  Fig. 
Ill  be  3  inches  long, 
the  long  arms  being 
2  inches,  and  the 
short  ones  1  inch. 


One  pound  at  A  will,  according  to  the  rule,  balance  2  at  B, 
and  2  at  B  will  balance  4  at  C,  and  4  at  C  will  balance  8  at 
D.  Therefore  1  pound  at  A  will  balance  8  pounds  at  D. 
Hence  it  is  evident  that  an  equilibrium  is  effected  when  the 
power  is  to  the  weight  as  the  product  of  all  the  short  arms 
is  to  the  product  of  all  the  long  arms.  The  compound  lever 
is  used  in  weighing  heavy  loads — as  hay,  coal,  etc.  Fig.  112 
shows  a  represen- 
tation of  the  ar- 
rangement. The 
load,  W,  stands 
on  a  platform, 
A  B,  which  rests 
upon  two  levers, 
E  D  and  E  C. 


The  long  arms  of 


Fiff.ll-2. 


THE    SIMPLE    MACHINES. 


143 


these  levers  are  E  G  and  E  F,  and  the  short  arms  are 
G  D  and  F  C.  The  ends  of  the  long  arms  press  upon  the 
fulcrum  of  the  lever,  H  I.  The  pressure  is  transmitted 
from  the  end  of  the  long  arm  by  the  rod,  I  K,  to  a  small 
lever,  K  L,  where  a  small  weight  or  power,  P,  balances  the 
weight  of  the  heavy  load,  W.  The  two  objects  secured 
by  this  arrangement  are  accuracy  and  the  occupation  of  a 
small  space. 

87.  Wheel  and  Axle. — The  next  of  the  simple  machines 
is  the  wheel  and  axle.  The  most  familiar  applications  of 
this  power  are  seen  in  drawing  water  and  in  raising  heavy 
articles  in  stores.  The  principle  of  this 
power  is  the  same  as  that  of  the  lever, 
as  may  be  shown  in  Fig.  113,  which 
represents  a  section  of  the  wheel  arid 
axle.  The  power,  P,  hangs  by  a  cord 
which  goes  round  the  wheel,  and  the 
weight,  W,  by  a  cord  around  the  axle. 
We  may  consider  the  power  as  pulling 
on  a  lever  represented  by  A  B,  the 
long  arm  of  which  is  A  C,  and  the 
short  arm  B  C.  You  see  that  the  wheel  and  axle,  then, 
may  be  viewed  as  a  constant  succession  of  levers,  and  it 

is  therefore  some- 
times called  the 
perpetual  lever. 
And  the  same 
rule  of  equilibri- 
um applies  here 
as  in  the  simple 
lever. 

In    the    com- 
mon    windlass 
Fig.  114.  •          the  power  is  ap- 


Fig.  113. 


144 


NATURAL  PHILOSOPHY. 


plied  to  a  winch  or  crank,  C  F,  Fig.  114,  instead  of  a  wheel. 

In  estimating  the  power  of  this  arrangement,  F  C  must  be 

considered  the  long  arm  of  the  lever,  and  half  the  diameter 

of  the  axle,  B  B,  as  its  short  arm. 

The  Capstan. — In  the  capstan,  represented  in  Figs.  115 

and  116,  the  axle  is  in  a  vertical  position.     The  top  of  it  is 

pierced  with  holes,  into  which  levers  are  introduced.     Fig. 

115  shows  the  instrument  as  commonly  used  in  moving 

buildings.  Some- 
times horse -power 
is  applied  at  the 
ends  of  the  levers. 
Great  power  is  ex- 
erted by  this  in- 
strument; but  we 
have  the  same  fact 
here  as  in  all  other 
cases  where  a  small 
force  produces  a 

great  effect — the  effect  is  slow,  and  the  force  passes  over  a 

great  space  in  producing  it.     The  moving  of  a  building  a 

foot  requires  many  circuits  of  the  horse  around  the  axle. 

The  capstan,  as  constructed  for  use  on  ships,  Fig.  116,  has 

a  circular  head,  with  many  holes  for  levers,  so  that  many 

men   can  work  together  in  raising 

a  heavy  anchor.     The  cable,  being 

wound  around  the  capstan  several 

times,  is  prevented  from  slipping 

by  friction;   and,  as  one  end  of 

the   cable    unwinds,   the    other   is 

wound  up.  Fig.  no. 

Fusee  of  a  Watch. — In  the  fusee  of  a  watch  we  have  a  wheel  and  axle 
of  a  peculiar  construction.  When  we  wind  up  a  watch,  the  chain  is  wound 
around  the  spiral  pathway  on  the  fusee,  B,  Fig.  117,  and,  at  the  same  time, 


Fig.  115. 


\ 


THE   SIMPLE   MACHINES. 


145 


Fig.  111. 


the  spring  is  coiled  up  tightly  in  the  round 
box,  A.  The  spring,  in  gradually  uncoiling 
itself,  turns  this  round  box  around,  and  thus 
pulls  upon  the  chain,  c,  making  the  fusee  to 
revolve,  and  thus  give  motion  to  other  parts 
of  the  machinery.  Now  the  spring,  in  its 
effort  to  uncoil,  acts  strongest  at  first;  and, 
therefore,  if  the  fusee  were  of  uniform  size,  the  watch  would  go  fastest 
when  first  wound  up,  and  go  gradually  slower  as  it  ran  down.  This  diffi- 
culty is  obviated  by  giving  the  power  a  small  wheel  to  pull  on  at  first,  and 
gradually  enlarging  the  wheel  as  the  spring  uncoils.  Because,  in  order  to 
produce  a  certain  effect  on  a  given  weight,  the  less  the  power,  the  longer 
must  be  the  arm  of  the  lever  on  which  the  power  acts. 

8'8.  Pulleys. — Another  simple  machine  is  the  pulley,  by 
which  masses  moving  with  different  velocities  may  be  con- 
nected, and  thus  forces  of  different  degrees  of 
intensity  balanced.  Pulleys  are  of  two  kinds, 
fixed  and  movable.  Fig.  118  represents  a 
tixed  pulley ;  the  wheel,  A  B,  has  a  groove  in 
its  circumference  which  prevents  the  rope 
from  slipping.  Its  action  may  be  conceived 
of  as  that  of  successive  levers  of  equal  arms, 
and,  therefore,  equilibrium 
requires  equality  of  power 
Fig.  us.  and  weigjjt<  Fixed  pulleys 

are  used  to  change  the  direction  of  forces, 
as  in  hoisting  sails 
on  board  ship.  By  a 
combination  of  two 
fixed  pulleys  a  hori- 
zontal force  may  be 
used  to  raise  weights 
vertically,  as  shown 
in  Fig.  119. 


Fte.  liu. 


Movable  Pulleys. —          Fig.  120. 


146 


NATURAL   PHILOSOPHY. 


Fig.  120  represents  a  movable  pulley.  In  this  case  it  is 
evident  that  the  force  exerted  by  the  weight  is  equally  di- 
vided between  the  ropes  S2  and  Sj.  A  movable  pulley  is 
sometimes  called  a  "  runner,"  and  a  fixed  pulley  is  often 
connected  with  it  in  order  to  give  the  desired  direction 
to  the  force.  Pulleys  are  often  connected  in  complicated 


Fig.  122. 


Fig.  123. 


THE    SIMPLE    MACHINES.  147 

Fig.  121  shows  a  system  of  fixed  and  movable 
pulleys;  the  weight,  q,  is  evidently  upheld  by  six  cords, 
which  divide  the  weight  equally  among  them.  If  q 
weigh  six  pounds,  equilibrium  will  be  obtained  by  mak- 
ing the  weight  of  p  equal  to  one  pound ;  at  the  same 
time,  it  must  be  remembered  that  p  will  move  six  feet 
while  q  moves  only  one.  Other  arrangements  of  pulleys 
are  shown  in  Figs.  122  and  123.  The  combination  of  pul- 
leys in  one  block  having  a  single  axle  (Fig.  122)  is  in  com- 
mon use. 

89.  The  Inclined  Plane. — This,  being  a  very  simple  con- 
trivance, is  much  used,  especially  when  heavy  bodies  are 
to  be  raised  to  only  a  small 
height,  as  in  moving  large  boxes 
and  hogsheads  into  stores.  The 
advantage  of  the  inclined  plane 
may  be  illustrated  by  Fig.  124. 
The  line  A  c  represents  an  in- 
clined  plane.  If  a  weight  be 

drawn  up  this  plane,  it  is  raised  only  the  height  I>  c. 
A  smaller  power  is  requisite  to  draw  the  weight  up  the 
plane  than  to  raise  it  perpendicularly ;  and  the  power 
necessary  will  be  the  less  the  longer  the  plane.  A  pow- 
er which  would  balance  a  weight  on  an  inclined  pi, -me 
would  be  to  the  weight  as  the  height  of  the  piano  to 
its  length.  Thus,  if  A  c  be  twice  as  long  as  B  c,  a 
weight  of  four  pounds  on  the  plane  may  be  balanced  by 
a  two -pound  weight  suspended  by  a  cord  passing  from 
the  weight  over  the  summit  of  the  plane.  A  flight  of 
stairs  is  constructed  on  the  principle  of  an  inclined  plane, 
the  projections  in  it  being  for  the  purpose  of  affording  a 
sure  footing  in  ascending  or  descending.  In  like  man- 
ner hogsheads  are  let  down  the  steps  of  a  cellar-way 
by  ropes,  and  it  makes  no  difference  in  the  principle 


148  NATURAL  PHILOSOPHY. 

of  the   operation  whether  planks  be   laid   on   the   steps 
or  not. 

It  is  supposed  that  the  immense  stones  in  the  pyramids  and  other 
massive  Egyptian  structures  were  put  into  their  position  by  means  of 
inclined  planes.  Roads,  when  not  level,  are  inclined  planes ;  and  the 
steeper  the  inclination,  the  more  power  is  required  to  draw  a  load  up  the 
road.  Great  mistakes  were  formerly  made  in  carrying  loads  straight  over 
high  hills.  Besides  failing  to  take  advantage  of  the  principles  of  the  in- 
clined plane,  in  many  cases  the  horse,  in  going  over  a  hill,  passes  over  quite 
as  much  space  as  he  would  if  the  road  were  made  to  go  round  the  base  of 
the  hill,  and  sometimes  even  more.  If  the  hill  were  a  perfect  hemisphere, 
a  road  over  it  would  be  just  equal  in  length  to  a  road  around  its  base  to 
the  opposite  point. 

;  90.  The  Wedge. — This  simple  device  may  be  considered  as 
two  inclined  planes  placed  with  their  bases  together,  as  seen 
in  Fig.  125.  Indeed,  sometimes  the 
wedge  has  one  side  only  inclined, 
it  being  but  half  of  the  ordinary 
wedge.  The  difference  between  the 
inclined  plane  and  the  wedge  in  op- 
eration is  that  in  the  first  the  inclined  plane  is  fixed,  and 
the  weight  is  made  to  move  up  along  its  surface ;  while  in 
the  latter  the  weight — that  is,  the  resistance — is  stationary, 
and  the  surface  of  the  plane  is  made  to  move  along  upon 
it.  The  power  of  the  wedge  is  estimated  just  like  that 
of  the  inclined  plane;  that  is,  by  comparing  the  thickness 
of  the  wedge  with  the  length  of  its  side.  The  less  the 
thickness  of  the  wedge  compared  with  its  length,  obviously 
the  more  powerful  is  the  wedge  as  a  penetrating  instru- 
ment. The  wedge  is  used  for  splitting  blocks  of  wood  and 
stone,  for  producing  great  pressures,  for  raising  heavy  bod- 
ies, etc.  All  cutting  and  piercing  instruments — knives,  ra- 
zors, axes,  needles,  pins,  nails,  etc. — act  on  the  principle  of 
the  wedge. 


THE    SIMPLE    MACHINES.  149 

Knives,  planes,  chisels,  etc.,  are  often  used  somewhat  in 
the  manner  of  a  saw,  by  drawing  their  edges  against  the 
object  to  be  cut,  at  the  same  time  that  the  pressure  ap- 
plied exerts  the  influence  of  a  wedge.  The  edge  of  a 
sharp  knife  examined  under  a  microscope  proves  to  be 
serrated.  The  sharpest  razor,  it  is  said,  may  be  pressed 
directly  against  the  hand  with  considerable  force  without 
cutting  the  skin,  while  if  drawn  ever  so  little  lengthwise  it 
will  inflict  a  wound. 

91.  The  Screw. — This  is  another  of  the  simple  machines. 
Its  principle  is  essentially  that  of  the  inclined  plane.  The 
"thread"  running  around  the  screw  is  an  inclined  plane 
which  is  spiral  instead  of  .straight;  and  the  corresponding 
part  in  the  nut  is  an  inclined  plane  running  in  the  opposite 
direction.  In  the  common  screw  the  nut  is  fixed,  and  the 
screw  is  made  to  play  up  and  down  in  it;  but  sometimes 
the  screw  is  fixed,  and  the  nut  is  made  to  play  around  it. 
The  screw  acts  like  a  wedge,  and  has  the  same  relation  to 
a  straight  wedge  that  a  road  winding  up  a  hill  has  to  a 
straight  road  of  the  same  length  and  rise.  Especially  does 
the  comparison  hold  when  the  screw  is  forced  into  wood ; 
the  wedge  goes  straight  into  the  wood,  but  the  edge  of  the 
screw's  thread  enters  the  wood  spirally. 

To  estimate  the  force  of  the  screw,  we  compare  the  length  of  one  turn 
of  the  thread  around  it  with  the  height  to  which  the  thread  rises  in  going 
round.  Let  a  6,  Fig.  126,  represent  one  turn  of 
the  thread,  b  c  the  height  to  which  it  goes.  It  is 
clear  from  the  figure  that  the  principle  which  ap- 
plies to  the  inclined  plane  and  to  the  wedge  applies 
here  also.  Since  the  less  the  height  of  the  plane, 
the  easier  it  is  for  a  weight  to  be  drawn  up  it ;  and 
since  the  less  the  depth  of  the  wedge,  the  less  is  it 
resisted;  therefore,  the  less  the  height  of  the  turn  of 
the  screw's  thread,  the  easier  is  it  to  move  the  screw, 
and  the  greater  the  force  which  it  exerts.  Hence  the 

G 


150 


NATURAL   PHILOSOPHY. 


Fig.  127. 


prodigious  power  of  a  screw  with  a 
thread  which  rises  very  slowly  in  its 
spiral  turns.  Screws  are  much  used 
when  great  pressure  is  required,  as 
in  pressing  oils  and  juices  from  vege- 
table substances,  in  compressing  cot- 
ton into  bales,  in  bringing  together 
with  firm  grasp  the  jaws  of  the  vice, 
etc.  In  turning  the  screw  a  bar  is 
used,  so  that  we  have  in  this  instru- 
ment the  combined  advantages  of  the 
screw  and  the  lever  (Fig.  127).  That 
you  may  have  some  idea  of  the  power 
of  these  two  instruments  acting  to- 
gether, we  will  state  an  imaginary 
case.  Suppose  it  is  desired  to  raise 


by  the  screw  a  weight  of  10,000  pounds.  Let  a  turn  of  the  screw  be  ten 
inches  long,  and  the  rise  be  but  one  inch.  Then,  so  far  as  the  screw  is 
concerned,  the  power  requisite  to  raise  the  10,000  pounds  will  be  1000— the 
ratio  of  the  height  of  the  thread's  turn  to  its  length.  But  the  power  of  the 
lever  is  yet  to  be  estimated.  Let  the  length  of  the  lever,  passed  through 
the  head  of  the  screw  so  that  it  is  equal  on  each  side,  be  thirty  inches. 
The  diameter  of  the  screw  is  about  three  inches,  or  one  tenth  of  the  diam- 
eter of  the  circle  described  by  the  end  of  the  lever.  It  will  take  but  a 
power  of  100  pounds  to  raise  the  weight,  the  ratio  of  the  radius  of  the 
screw  to  half  the  length  of  the  lever. 

92.  Other  Simple  Machines. — When  the  old  idea  prevail- 
ed that  the  simple  machines  were  but  six  in  number,  it  was 
shown  that  these  could  be  reduced  practically  to  three,  for 
the  wedge  and  the  screw  are  modifications  of  the  inclined 
plane,  and  the  wheel  and  axle  is  a  modification  of  the 
lever.  As  already  mentioned,  however,  there  is  no  neces- 
sity for  limiting  to  so  small  a  number  the  simple  machines, 
provided  we  regard  them  as  simply  modifiers  of  motive 
power.  The  toggle-joint  is  a  simple  machine,  in  which  the 
connected  parts  are  arranged  so  as  to  move  with  different 
velocities.  It  is  used  to  raise  carriage  -tops,  and,  when 


THE    SIMPLE    MACHINES.  151 

made  of  immense  strength,  to  exert  great  pressure 
through  a  small  space,  as  in  shearing  and  punch- 
ing iron.  Fig.  128  represents,  in  skeleton  form,  a 
toggle-joint.  Machines  which  change  the  direc- 
tion of  the  force  applied  are  very  numerous,  but 
a  study  of  them  belongs  properly  to  higher  me- 
chanics. We  see  their  applications  in  compli- 
cated machines'  in  common  use  —  the  printing- 
press,  sewing-machine,  steam-engines,  locomo- 

Jj'ig. 

tives,  and  many  others. 

93.  The  Pendulum. — Certain  mechanical  contrivances  are 
designed  for  the  modification  of  mere  motion,  without  any 
reference  to  the  transmission  of  forces :  such  are  watches, 
clocks,  and  timepieces  of  every  description.  These  instru- 
ments have  for  their  object  the  production  of  a  perfectly 
uniform  motion  with  a  view  to  the  measurement  of  time. 
The  motion-regulator  by  which  clocks  are  controlled — the 
pendulum — demands  a  somewhat  extended  notice. 

Various  modes  of  measuring  time  have  been  adopted  by 
mankind.  At  first,  time  was  inaccurately  divided  by  mere- 
ly observing  the  sun.  But  after  a  while  man  resorted  to 
various  contrivances  to  measure  short  periods  of  time  with 
accuracy.  All  of  these  depend  upon  the  uniformity  of  mo- 
tion alone.  The  sundial  measures  time  by  the  uniform 
movement  of  the  shadow  on  its  face,  caused  by  the  uni- 
form movement  of  the  earth  in  relation  to  the  sun.  The 
hour-glass  measures  time  by  the  uniform  fall  of  sand  pro- 
duced by  the  attraction  of  gravitation.  The  best  measure- 
ment of  time  is  by  the  comparatively  modern  invention 
of  clocks  and  watches,  in  which  time  is  divided  into  very 
minute  periods  by  the  uniform  motion  of  the  pendulum  or 
the  balance-wheel.  The  pendulum  furnishes  an  interesting 
example  of  motion  sustained  by  the  influence  of  gravity. 
It  was  not  till  the  time  of  Galileo,  less  than  three  centuries 


152  NATURAL   PHILOSOPHY. 

ago,  that  its  operation  was  understood  and  appropriated  to 
the  measurement  of  time.  He  observed  that  a  chandelier 
hanging  from  the  lofty  ceiling  of  a  cathedral  in  Pisa  vi- 
brated very  long  and  uniformly  when  accidentally  agi- 
tated, and  the  thought  of  the  philosopher  evolved  from 
this  phenomenon  the  most  important  results.  Though  it 
had  been  before  men's  eyes  in  some  shape  or  other  since 
the  creation,  it  was  reserved  for  Galileo  to  observe  its 
significance,  and  to  pave  the  way  which  has  led  to  tluj 
use  of  the  pendulum  as  man's  time-keeper  over  the  whole 
earth. 

Explanation  of  its  Operation. — A  pendulum  commonly 
consists  of  a  ball  or  weight  at  the  end  of  a  rod  suspend- 
(i  ed  so  as  to  vibrate  with  little 

s  friction  at  the  point  of  the  sus- 

\  pension.     Let  a  b,  Fig.  129,  rep- 

\  resent  such  a  pendulum.    When 

\  it  is  at  rest,  it  makes  a  plumb- 

line  hanging  towards  the  centre 

of  the  earth.     If  it  be  raised  to 
dr*\ 
ly/fr  c  and  be  left  to  foil,  the  force  of 

Fig- 129-  gravity  will  not  only  carry  it  to 

&,  but,  by  the  accumulated  momentum  acquired  in  its  de- 
scent, gravity  will  carry  it  to  d.  The  same  would  be  true 
of  its  return  from  d.  And  it  would  vibrate  forever  in  this 
way  if  it  could  be  entirely  freed  from  the  resistance  of  the 
air  and  from  friction.  But,  as  it  is,  the  pendulum,  left  to 
itself,  gradually  loses  its  motion  from  these  obstacles.  In 
the  common  clock,  the  office  of  the  weight  ,is  to  counteract 
the  influence  of  these  obstacles  and  keep  the  pendulum  vi- 
brating. In  the  watch,  the  mainspring  performs  the  same 
office  to  the  balance-whgel. 

The  times  of  the  vibrations  of  a  pendulum  are  nearly 
equal,  whether  the  arc  it  describes  be  great  or  small;  for 


THE    SIMPLE    MACHINES.  153 

when  the  vibration  is  a  large 
one,  the  velocity  which  the 
pendulum  acquires  in  falling  is 
greater  than  when  the  vibration 
is  of  small  extent.  The  reason 
is,  that  the  higher  it  rises,  the 
steeper  the  beginning  of  its  de- 
scent. Thus  a  c,  Fig.  130,  is 

steeper  than  c  b.  The  longer  a  pendulum,  the  longer  time 
does  its  vibration  occupy.  It  requires  a  pendulum  of 
the  length  of  a  little  over  thirty-nine  inches  (39.13")  to 
vibrate  once  every  second.  Cold  weather,  by  contracting 
the  pendulum,  makes  it  vibrate  quicker  than  in  summer, 
and  so  makes  the  clock  go  faster.  Various  contrivances 
have  been  resorted  to  in  order  to  counteract  the  variation 
of  length  in  pendulums  by  heat  and  cold,  one  of  which  we 
will  describe  in  the  chapter  on  heat  (§  167). 

The  popular  idea  that  a  heavy  body  falls  quicker  than  a 
light  one  is  dispelled  by  the  fact  that  pendulums  vibrate 
equally  fast  or  slow,  no  matter  of  what  material  they  are 
constructed.  Similar  pendulums  of  lead,  glass,  iron,  or 
wood,  or  even  a  hollow  ball,  vibrate  at  the  same  rate. 

94.  Friction. — Friction  is  the  resistance  offered  to  a  body 
by  the  surface  on  which  it  moves.  It  seems  to  arise  from 
adhesive  attraction  between  the  touching  substances  and 
from  the  roughness  of  their  surfaces.  The  rougher  the 
surfaces  brought  in  contact,  the  greater  the  friction,  the 
little  cavities  and  projections  fitting  into  each  other  and 
necessitating  a  certain  force  to  raise  the  projections  on  one 
surface  from  the  cavities  in  the  other.  Substances  which 
appear  quite  smooth  to  the  naked  eye,  as  polished  steel, 
nevertheless  exhibit  inequalities  of  surface  when  examined 
under  the  microscope. 

Friction  acts  as  an  obstacle  to  motion.     When  we  roll 


\ 


154 


NATURAL   PHILOSOPHY. 


a  ball,  the  rougher  the  surface  on  which  it  is  rolled,  the 
greater  the  friction  and  the  sooner  the  ball  stops.  Ma- 
chinery, no  matter  how  carefully  constructed,  suffers  a 
waste  of  power  through  friction,  even  to  the  extent  of  a 
third  or  a  half  of  the  force  applied.  Hence  various  expe- 
dients are  resorted  to  for  the  purpose  of  diminishing  fric- 
tion, such  as  polishing  the  rubbing  surfaces,  and  oiling  or 
otherwise  lubricating  them.  Using  wheels,  as  on  car- 
riages, effects  the  same  end.  A  heavy  load,  which  the 
most  powerful  horse  could  not  move  if  placed  on  a  "  stone- 
boat,"  is  readily  drawn  along  in  a  wheeled  vehicle.  Cast- 
ers attached  to  household  furniture  prevent  the  friction 
arising  from  dragging  it  over  carpets. 

The  very  fact  that  rapidity  of  motion  is  lessened  by  fric- 
tion is  in  some  cases  of  the  greatest  importance  to  us. 
"  But  for  friction,  men  walking  on  the  ground  or  pavement 
would  always  be  as  if  walking  on  ice ;  and  our  rivers  that 
now  flow  so  calmly  would  all  be  rapid  torrents.  It  is  fric- 
tion which  retains  all  loose  objects  on  earth  in  the  situa- 
tions in  which,  for  convenience,  men  choose  to  place  them — 
the  furniture  of  a  house,  the  contents  of  libraries,  museums, 
etc.  Friction,  therefore,  is  essential 
to  our  existence." 


Fig.  131  illustrates  a  simple  method  of 
taking  advantage  of  friction.  The  weight,  P, 
can  be  lowered  gradually,  and  with  compara- 
tive ease,  by  wrapping  the  rope  about  the  cyl- 
inder, A  C  B  ;  whereas,  without  the  friction 
of  the  rope,  the  force  of  gravity  would  be  be- 
yond the  control  of  the  power,  Q.  In  this 
way  heavy  casks  which  are  otherwise  unman- 
ageable are  lowered  into  cellars. — Fig.  132 
shows  us  at  once  the  advantages  of  friction, 
and  a  means  of  overcoming  its  disadvantages. 
The  block  of  stone,  Q,  is  supported  by  roll- 


Fig.  131. 


THE    SIMPLE    MACHINES.  155 


Fig. 132. 

ers,  in  order  to  overcome  the  friction  of  its  surface  on  the  ground ;  and 
by  means  of  the  friction  between  the  rope,  B,  and  the  axle,  O  C,  the  rope 
is  prevented  from  slipping  when  power  for  moving  the  block,  Q,  is  ap- 
plied to  the  lever,  A. 

The  friction  of  the  driving-wheels  of  a  locomotive  upon  the  rails  pre- 
vents them  from  slipping.  In  this  case  the  wheel  pushes  backward  on 
the  rail  at  each  successive  point  of  contact.  To  make  this  clear,  suppose 
a  common  wheel  be  deprived  of  its  rim  and  be  made  to  revolve  on  the  ends 
of  its  spokes.  The  end  of  each  spoke  gives  a  backward  push  as  it  strikes 
the  ground.  Now  the  rim  of  a  wheel  makes  the  same  pushes,  but  they  arc 
more  numerous — they  are  continuous,  being  made  by  all  the  successive 
points  in  the  rim.  Sometimes  the  rails  of  a  railroad  are  too  smooth,  from 
frost  or  some  other  cause,  and  then  sand  is  thrown  upon  them  to  enable  the 
locomotive  to  start.  The  sand  serves  to  prevent  the  wheels  from  sliding  by 
enabling  them  to  get  some  hold  upon  the  rails  in  their  backward  pushes. 

95.  The  Real  Advantages  of  Machinery. — If  there  is,  then, 
no  saving,  but  a  loss,  of  power  in  tools  and  machinery, 
what,  let  us  inquire,  are  their  advantages? 

If  one  man  can  do  alone  by  the  aid  of  some  instrument 
that  which  would  otherwise  require  the  exertion  of  many 
men,  although  slow  in  accomplishing.it,  yet  it  is  a  great 
advantage.  Thus,  one  man  with  a  lever  can  move  a  stone 
which  it  would  require  perhaps  thirty  men  to  move  with- 
out it;  and  though  it  take  him  thirty  times  as  long,  it 
saves  him  the  trouble  of  getting  a  company  of  men  to  help 
him.  So  if  a  man  can  raise  his  goods  by  a  wheel  and  axle 
to  the  upper  loft  of  his  store,  though  he  raise  them  more 
slowly  than  several  men  would  lift  them  directly  by  ropes, 


156  NATURAL   PHILOSOPHY. 

it  is  an  advantage  to  him,  since  it  saves  the  hiring  of  a 
number  of  laborers.  It  must,  however,  be  remembered 
that  what  is  gained  in  amount  of  work  is  lost  in  time,  and 
what  is  gained  in  time  by  using  any  machine  is  lost  in  the 
amount  of  work. 

Another  advantage  is  that,  in  applying  the  force,  inter- 
vals of  rest  may  be  secured  without  any  loss.  This  is 
obvious  in  the  case  of  the  pulley,  but  still  more  so  in  the 
case  of  the  screw.  It  is  friction  in  both  these  cases  which 
enables  the  workman  to  rest.  It  saves  to  him  all  that  he 
has  gained  by  opposing  any  tendency  to  slip  back.  The 
same  thing  is  true  of  the  wedge.  When  this  is  driven  into 
Avood,  it  remains  fixed  because  prevented  from  returning 
by  the  friction  of  the  wood  against  its  sides.  It  is  the 
same  cause  which  holds  a  nail  in  its  place,  and  opposes  any 
effort  to  draw  it  out.  In  driving  the  wedge,  the  workman 
can  have  as  long  intervals  as  he  pleases  between  his  blows, 
because  friction  saves  all  that  is  gained.  This  effect  is 
very  well  exemplified  in  the  capstan,  Fig.  115.  It  requires 
but  little  exertion  of  the  man  who  sits  there  to  hold  the 
rope,  because  the  few  turns  of  it  around  the  axle  prevent 
its  slipping  easily. 

A  third  advantage  which  often  attends  the  use  of  tools 
and  machines  is  that  force  may  be  made  to  produce  motion 
at  various  distances,  in  various  directions,  and  in  various 
degrees  of  velocity.  Thus,  as  to  distance,  a  man  standing 
on  the  ground  can  raise  a  weight  to  the  top  of  a  house  by 
a  pulley.  A  water-wheel  may  by  the  connections  of  ma- 
chinery produce  motion  at  considerable  distances  from  it. 
Then,  as  to  direction,  horizontal  motion  may  be  converted 
into  vertical,  rotary  into  straight,  etc.  The  velocity  of 
motion  is  generally  varied  by  cog-wheels.  Thus,  a  wheel 
of  60  cogs,  revolving  once  in  a  minute,  playing  on  a  wheel 
of  10  cogs,  will  make  it  revolve  once  in  6  seconds. 


THE    SIMPLE    MACHINES.  157 

Another  advantage  of  tools  and  machines  is  that  they  secure  a  better 
mode  of  applying  power  than  we  otherwise  could  have.  Thus,  when  sev- 
eral men  are  pulling  on  a  rope,  much  power  is  lost  by  their  pulling  irregu- 
larly, a  difficulty  which  is  removed  by  the  pulley.  The  same  can  be  said 
of  applying  pressure  by  the  screw.  One  man  presses  more  steadily,  and 
therefore  more  effectually,  than  fifty  men  would  without  the  screw.  The 
arrangements  of  tools  and  machines  are  so  made  as  to  provide  convenient 
ways  of  applying  our  strength.  An  instrument,  for  example,  for  moving  a 
weight  by  hand  is  so  shaped  as  to  hold  the  weight  well,  and  also  to  afford 
a  good  handle  for  the  hand  to  grasp.  The  common  claw-hammer  is  a  very 
good  illustration.  We  grasp  the  nail  by  an  iron  claw ;  with 
the  handle  we  can  apply  not  merely  the  force  of  the  hand,  but 
that  of  the  whole  arm,  and  then  we  have  the  immense  lever 
power  of  the  instrument.  We  have  a  good  illustration  of 
convenience  in  an  instrument  in  what  is  called  a  Lewis,  rep- 
resented in  Fig.  133.  It  is  used  for  raising  blocks  of  stone  in 
building.  It  has  three  parts,  ABC.  It  is  used  in  this  way : 
A  hole  of  the  same  shape  as  the  instrument  is  made  in  the  Fi£- 133> 
upper  part  of  the  block  of  stone  to  be  raised  ;  then  A  and  C  arc  inserted, 
and  B  is  pushed  in  between  them.  With  the  ring,  D,  bolted  through  the 
instrument,  the  stone  is  raised  to  its  place  by  the  ordinary  machinery. 
The  principle  of  the  instrument,  you  see,  is  that  of  the  wedge. 

96.  Man  a  Tool-making  Animal. — Though  there  is  no  ac- 
tual saving  of  power  in  the  tools  and  machines  which  man 
uses,  yet  so  great  are  the  advantages  which  he  reaps  from 
them  that,  more  than  two  thousand  years  ago,  a  philos- 
opher thought  that  man  could  not  be  better  distinguished 
from  brutes  than  by  calling  him  a  tool -making  animal. 
If  the  distinction  was  so  striking  in  the  time  of  Aristotle, 
when  tools  and  machines  were  so  few  "in  number  and  so 
rudely  contrived,  and  so  few  of  the  sources  of  power  were 
appropriated  by  man  to  his  use,  how  much  more  striking  is 
it  now,  with  all  the  variety  and  perfection  of  instruments 
and  machinery,  and  with  the  ever-extending  appropriation 
of  the  sources  of  power  furnished  by  the  elements  !  The 
power  which  air  and  water  and  gravitation  supply  is  ap- 
plied constantly  with  more  and  more  variety  and  effect ; 

G  2 


158  NATUKAL  PHILOSOPHY. 

and  the   appropriation  of  that  mighty  source  of  power, 
Bteam,  is  wholly  a  modern  invention. 


QUESTIONS. 

81.  What  is  said  of  the  forces  of  nature  and  of  machines?  Name  the 
six  simple  machines.  Why  is  the  term  mechanical  powers,  formerly  ap- 
plied to  them,  erroneous  ?  Explain  the  terms  power,  weight,  and  fulcrum. 
— 82.  How  are  levers  classified?  What  is  the  lever  of  the  first  kind? 
Illustrate  its  uses.  What  is  said  of  steelyards  and  the  balance?  Give 
other  examples  of  this  kind  of  lever. — 83.  Show  that  there  is  no  gain  of 
power  in  this  lever.  What  is  said  of  the  seesaw  ?  What  boast  did  Ar- 
chimedes make  ? — 84.  What  is  a  lever  of  the  second  kind  ?  Give  examples 
of  its  use. — 85.  What  is  a  lever  of  the  third  kind  ?  Give  examples. — 80. 
What  constitutes  a  compound  lever?  Explain  its  use  in  platform  scales. 
— 87.  Explain  the  action  of  the  wheel  and  axle.  What  is  said  of  the 
windlass  ?  What  of  the  capstan  ?  Explain  the  construction  of  the  fusee 
of  a  watch. — 88.  What  is  the  advantage  of  the  pulley  ?  Describe  the  fixed 
pulley  and  illustrate  its  uses.  Describe  a  movable  pulley.  Explain  some 
of  the  arrangements  of  pulleys. — 89.  Illustrate  the  mechanical  advantage 
of  the  inclined  plane.  What  is  said  of  railways  ? — 90.  What  is  a  wedge  ? 
Give  examples  of  its  uses. — 91.  Upon  what  principle  does  the  screw  work? 
How  is  its  force  estimated?  Give  an  example. — 92.  Describe  some  other 
simple  machines. — 93.  What  is  said  of  the  pendulum?  Who  first  made 
use  of  it  for  the  measurement  of  time?  Explain  its  operation. — 94.  What 
is  meant  by  friction  ?  What  is  its  effect  in  machinery  ?  How  may  it  be 
overcome?  Illustrate  the  uses  made  of  friction. — 95.  What  is  the  first 
advantage  of  the  simple  machines  which  is  mentioned  ?  Give  the  illustra- 
tions. What  is  the  second  advantage  ?  Give  the  illustrations.  What  is 
the  third  advantage  ?  Give  examples.  How  is  the  velocity  of  motion  in 
machinery  usually  varied  ?  What  is  the  fourth  advantage  ?  Mention  ex- 
amples. Describe  the  instrument  called  a  Lewis.— 9G.  What  is  said  of 
the  title  by  which  Aristotle  distinguished  man  from  other  animals  ? 


HYDROSTATICS.  159 


CHAPTER  X. 

HYDROSTATICS. 

97.  What   Hydrostatics    Teaches. — A    single    substance 
may,  as  we  have  seen,  exist  in  three  forms — solid,  liquid, 
and  gaseous — and  these  forms  are  distinguished  one  from 
another  by  the  difference  in  the  mobility,  or  flow,  of  the  par- 
ticles composing  the  substance.     In  a  solid  the  particles 
are  comparatively  rigid,  the  force  of  cohesion  being  strong ; 
in  a  liquid  or  a  gas  they  are  not  in  such  close  contact,  and 
are  freer  to  move  about  each  other.     A  body  in  either  the 
liquid  or  the  gaseous  state  is  called  a  fluid,  on  account  of 
ilwflow  of  the  particles.     A  very  important  branch  of  Nat- 
ural Philosophy  relates  to  the  pressure,  motion,  and  other 
phenomena  of  fluids,  which  for  convenience  is  considered 
under  three  heads,  viz. :  Hydrostatics,  which  treats  of  the 
pressure  of  liquids ;  Hydraulics,  which  treats  of  their  mo- 
tion ;  and  Pneumatics,  which  treats  of  the  same  phenomena 
in  air  and  gases. 

In  order  to  understand  the  phenomena  of  Hydrostatics, 
you  must  bear  in  mind  that  they  result  from  the  influence 
of  the  attraction  of  the  earth  upon  liquids,  and  that  they 
depend  upon  the  two  great  characteristics  of  liquids,  their 
mobility  and  incompressibility  (§  17). 

98.  Level  Surface  of  Liquids. — Owing  to  the  perfect  mo- 
bility of  the  particles  among  each  other,  and  their  being 
equally  attracted  towards  the  centre  of  the  earth,  liquids 
at  rest  assume  a  level  surface.     The  particles  forming  the 
surface  may  be  regarded  as  the  tops  of  so  many  columns 
of  particles  supported  by  a  uniform  resistance  or  pressure 


160  NATURAL  PHILOSOPHY. 

below;  for  no  particle  below  can  be  at  rest  unless  urged 
equally  in  all  directions,  and  therefore  all  the  particles,  at 
any  one  level,  which,  by  equally  urging  one  another,  keep 
themselves  at  rest,  must  be  bearing  the  weight  of  equal 
columns.  Thus,  a  higher  column,  however  produced,  must 
sink  and  a  lower  one  must  rise  until  just  balanced  by  those 
around ;  that  is,  until  all  become  alike.  The  particles  of 
water  may  be  compared  to  shot ;  if  you  place  shot  in  a 
box  and  heap  them  up  in  any  portion  of  the  surface,  on 
shaking  the  box  those  that  are  highest  will  roll  down,  and 
a  level  surface  will  result.  They  would  do  this  without 
agitation  if  they  were  as  free  to  move  among  themselves 
as  are  the  particles  of  water.  If  a  microscope  could  be 
made  strong  enough  to  distinguish  the  shape  of  the  parti- 
cles of  water,  the  surface  might  possibly  appear  like  the 
level  surface  of  shot  in  a  vessel.  But  the  particles  of  water 
are  so  exceedingly  minute  that  the  surface  of  water,  when 
entirely  free  from  agitation,  is  so  smooth  as  to  constitute  a 
perfect  mirror,  often  feasting  our  eyes  with  another  world 
of  beauty  as  we  look  down  into  its  quiet  depths.  Strictly 
speaking,  the  surface  of  a  liquid  is  not  level  or  horizontal ; 
being  parallel  to  the  surface  of  the  earth,  it  forms  a  curve, 
but  this  curved  surface  is  of  so  great  a  radius. that  it  cannot 
be  perceived  unless  we  take  into  view  a  very  large  surface, 
as  the  ocean.  Here  it  is  very  manifest ;  for  whenever  a  ship 
sails  out  of  port,  the  topmost  sail  is  the  last  thing  seen  from 
the  shore,  the  rest  of  the  ship  being  concealed  by  the  water 
rounded  up  between  it  and  the  observer.  This  is  illustrated 
in  Fig.  134. 

ML 


HYDROSTATICS.  161 

If  the  earth  had  no  elevations  of  land,  or  if  there  were  water  enough  to 
cover  them,  the  water  would  make  a  perfectly  globular  covering  for  the 
earth,  being  held  to  it  by  the  force  of  attraction.  The  reason  for  this  is 
precisely  the  same  as  was  given  in  §  31  for  the  disposition  of  a  drop  of 
liquid  to  take  the  globular  form.  As  in  that  case,  so  in  this,  it  can  be  de- 
monstrated that  each  particle  is  attracted  towards  a  common  centre,  and 
that  this  will  produce  in  the  freely  moving  particles  a  uniformly  rounded 
surface.  What  could  thus  be  shown  to  be  true  if  the  earth  were  wholly 
covered  with  water  is  true  of  the  portions  of  water  which  now  fill  up  the 
depressions  in  the  earth's  crust. 

Spirit-Level. — What  is  commonly  called  a  perfectly  level 
surface  is,  then,  one  in  which  every  point  is  equally  distant 
from  the  centre  of  the  earth,  and  is  therefore  really  a  spher- 
ical surface.  But  the  sphere  is  so  large  that  any  very  small 
portion  of  it  may  be  considered,  for  all  practical  purposes, 
a  perfect  plane.  A  hoop  surrounding  the  earth  would  bend 
about  four  inches  in  every  mile.  In  cutting  a  canal,  there- 
fore, there  is  a  variation  in  this  proportion  from  a  straight 
level  line.  Since  the  variation  is  but  an  inch  in  a  fourth  of 
a  mile,  it  is  of  no  account  in  taking  the  level  for  buildings. 
Levels  are  ascertained  by  what  is  called  a  spirit-level.  This 

consists  of  a  closed  glass  tube, g 

Fig.  135,  nearly  filled  with  alco- 
hoi.     This  liquid  is  used  in  pref-  Fig.  135. 

erence  to  water  because  it  never  freezes  and  is  more  mo- 
bile. The  space  .not  filled  with  alcohol  is  occupied  by  air. 
The  tube  is  placed  in  a  wooden  box  for  convenience  and 
security,  there  being  an  opening  in  the  box  at  a.  Now, 
when  the  box  with  its  glass  tube  is  perfectly  level,  the 
bubble  of  air  will  be  seen  in  the  middle  at  a\  but  if  one  end 
be  higher  than  the  other,  the  bubble  will  be  at  or  towards 
that  end.  This  simple  instrument  is  used  by  masons  and 
carpenters  for  the  purpose  of  levelling  walls  or  floors  of 
buildings,  by  engineers  in  surveying,  and  by  others. 

99.  Flow  of  Rivers.  —  If  a  trough  be  exactly  level,  the 


162  NATURAL   PHILOSOPHY. 

water  will  be  of  the  same  depth  at  one  end  as  at  the  other, 
for  the  surface  of  the  water  at  both  ends  will  be  at  the 
same  distance  from  the  centre  of  the  earth.  But  if  one  end 
be  raised,  the  water  will  become  deeper  at  the  other  end. 
If  it  were  not  so,  the  surface  at  the  two  ends  would  not  be 
at  the  same  distance  from  the  centre  of  the  earth.  Now, 
if  water  run  in  at  the  tipper  end  of  an  inclined  trough 
and  out  at  the  lower,  we  have  an  illustration  of  what  takes 
place  in  all  rivers — the  water  is  in  constant  motion.  A 
very  slight  incline  gives  a  flow  to  water,  for  the  particles 
are  so  mobile  that,  in  obedience  to  the  force  of  gravitation, 
they  descend  the  inclined  plane  to  seek  a  level.  Three 
inches  per  mile  in  a  smooth  straight  channel  gives  a  veloc- 
ity of  about  three  miles  an  hour.  The  Ganges,  which  gath- 
ers the  waters  of  the  lofty  Himalaya  Mountains,  in  running 
1800  miles  falls  only  800  feet;  and  to  fall  gradually  these 
800  feet  in  its  long  course,  the  water  takes  nearly  a  month. 
The  gigantic  Rio  de  la  Plata  has  so  gentle  a  descent  to  the 
ocean  that,  in  Paraguay,  1500  miles  from  its  mouth,  large 
ships  arrive  which  have  sailed  against  the  current  all  the 
way  by  the  force  of  the  wind  alone. 

100.  How  some  Rivers  have  been  Made.  —  Changes  are  con- 
stantly'produced  in  the  earth  by  the  disposition  of  water  to  seek- a  level. 
In  doing  this  the  water  carries  solid  substances  of  various  kinds  from  ele- 
vated places  into  depressed  ones,  tending  to  fill  up  the  latter.  New  chan- 
nels are  also  sometimes  made  by  the  water.  The  boy  who  makes  a  little 
pond  with  his  mud-dam,  and  lets  the  water  overflow  from  it  into  another 
pond  on  a  lower  level,  as  he  sees  a  channel  worked  by  the  water  between 
the  two  ponds  becoming  larger  and  larger,  witnesses  a  fair  representation 
on  a  small  scale  of  some  extensive  changes  which  have,  in  ages  past,  taken 
place  in  some  parts  of  the  earth.  It  is  supposed,  and  with  good  reason, 
that  many  rivers  had  their  origin  in  the  way  above  indicated.  For  exam- 
ple, where  the  Danube  runs  its  long  course  there  was  once  a  chain  of  lakes. 
These  becoming  connected  together  by  their  overflow,  the  channels  cut  be- 
tween them  by  the  water  continually  became  larger,  until  at  length  there 
was  one  long,  deep,  and  broad  channel,  the  river ;  while  the  lakes  became 


HYDROSTATICS. 


163 


dry,  and  constituted  the  fertile  valley  through  which  that  noble  river  runs 
to  empty  into  the  Black  Sea.  It  is  said  that  a  similar  process  is  mani- 
festly going  on  in  the  Lake  of  Geneva,  the  outlet  of  it  becoming  continu- 
ally broader,  while  the  washing  from  the  neighboring  hills  and  mountains 
is  filling  up  the  lake.  Towns  that  a  century  ago  lay  directly  upon  the  bor- 
ders of  the  lake  now  have  gardens  and  fields  between  them  and  the  shore ; 
and  Dr.  Arnott  says,  "If  the  town  of  Geneva  last  long  enough,  its  in- 
habitants will  have  to  speak  of  the  river  threading  the  neighboring  valley, 
instead  of  the  picturesque  lake  which  now  fills  it." 

101.  Canals. — The  management  of  the  locks  of  a  canal  is 
in  conformity  with  the  disposition  of  water  to  seek  a  level. 
A  ground  view  of  one  lock  and  a  part  of  two  adjacent  locks 
is  given  in  Fig.  136.  The  lock  C  has  two  pairs  of  flood- 


gates,  D  D  and  E  E.  The  water  in  A  is  higher  than  in  C, 
but  the  level  is  the  same  in  C  and  B,  because  the  gates, 
E  E,  are  open.  Suppose  there  is  a  boat  in  the  lock  B 
that  you  wish  to  get  into  the  lock  A.  It  must  be  floated 
into  the  lock  C,  and  the  gates  E  E  must  be  closed.  The 
water  may  now  be  made  to  flow  from  *the  higher  level,  A, 
into  C,  till  the  level  is  the  same  in  both  A  and  C.  But 
this  cannot  be  done  by  opening  the  gates  D  D,  for  the 
pressure  of  such  a  height  of  water  in  the  lock  A  would 
make  it  difficult,  perhaps  impossible,  to  do  this;  and,  be- 
sides, if  it  could  be  done,  the  rapid  rush  of  water  into  C 
would  flood  the  boat  lying  there.  The  discharge  is  there- 
fore effected  by  openings  in  the  lower  part  of  the  gates 


164 


NATURAL   PHILOSOPHY. 


D  D.  These  openings  are  covered  by  sliding  shutters, 
which  are  raised  by  racks  and  pinions,  as  represented  in 
Fijj.  137.  When  the  water  has  become  of  the  same  level 


Fig.  13T. 

in  A  and  C,  the  gates  D  D  can  be  easily  opened,  and  the 
boat  may  be  floated  from  C  into  A.  If  a  boat  is  to  pass 
downward  in  the  locks,  the  process  described  must  be  re- 
versed. 

Canals  are  also  extensively  used  for  supplying  water  by  side  openings  to 
turn  water-wheels  for  the  working  of  machinery.  The  water  turns  the 
wheel  by  the  force  which  gravitation  gives  it  as  it  descends  from  the  level 
of  the  canal  to  the  level  of  the  river.  One  of  the  grandest  canals  in  the  world 
is  that  cutting  through  the  Isthmus  of  Suez,  constructed  in  18(54-69  under 
the  direction  of  a  French  engineer,  M.  I)e  Lesseps.  It  is  about  one  hun- 
dred miles  in  length,  and  connects  the  waters  of  the  Mediterranean  with 
those  of  the  Red  Sea.  The  importance  of  canals  as  a  means  of  transporta- 
tion of  heavy  goods  is  evident  from  the  fact  that  a  horse  which  can  draw 
but  one  ton  on  our  best  roads  can  draw  thirty  with  the  same  speed  in  a 
canal-boat. 

102.  Other  Illustrations. — We  see  the  tendency  of  liquids 
to  rise  to  the  same  level  in  other  ways.  In  a  coffee-pot 


HYDROSTATICS. 


165 


the  liquid  has  the  same  level  in  the  spout  as  in  the  vessel 
itself,  whatever  may  be  its  position,  as  seen  in  Fig.  138. 


If  it  be  turned  up  so  far  that  the  level  of  the  fluid  in  the 
vessel  is  higher  than  the  outlet  of  the  spout,  the  fluid  runs 
out.  If  two  reservoirs  of  water  be  connected,  the  water 
will  stand  at  the  same  height  in  both,  whatever  the  dis- 
tance between  them  may  be.  In  the  aqueduct  pipes  that 
extend  from  a  reservoir,  the  water  will  rise  as  high  as 
the  surface  of  the  water  in  the  reservoir  itself.  If  the 
outlets  of  the  pipes  be  lower  than  this  level,  the  water 
will  run  from  them,  as  in  the  case  with  the  coifee.  The 
cause  of  these  and  similar  facts  is  the  same  as  that  of  the 
level  surface  in  vessels  and  reservoirs — the  action  of  gravi- 
tation. This  may  be  made  plain 
by  Fig.  139.  Let  the  figure  rep- 
resent the  section  of  a  vessel 
with  divisions  of  different  de- 
grees of  thickness,  these  divi- 
sions, however,  not  extending 
to  the  bottom  of  the  vessel. 
Water  in  this  will  stand  at  the 
same  level  in  the  different  com- 


partments, just  as  it  would  if  the  vessel  had  no  such  divi- 
sions. This  is  simply  because  the  attraction  of  the  earth 
acts  upon  the  water  in  the  same  way  with  the  divisions  as 


166 


NATURAL   PHILOSOPHY. 


Fig.  140. 


without  them.  And  you  can  see  that  it  will  make  no  dif- 
ference whether  these  divisions  be  thick  or  thin,  or  whether 
the  apartments  be  near  together  or  far  apart,  as  in  the  case 
when  branch  pipes  extend  from  a  reservoir.  A  branch 
pipe  may  be  considered  as  having  the  same  relation  to  the 
reservoir  as  one  of  the  narrow  compartments  in  the  figure 

has  to  the  rest  of  the  vessel. 
The  result  is  not  at  all  affected 
by  either  the  size  or  form  of  the 
tubes  that  may  be  connected 
with  a  common  reservoir:  a  fluid 
will  stand  at  the  same  height  in 
all.  Thus  we  have,  in  Fig.  140, 
tubes  of  various  size  and  shape, 
A,  B,  C,  connected  by  the  pipe, 
m  ?i,  with  a  reservoir,  D ;  and  if 
water  be  poured  into  the  latter,  it  will  rise  to  the  same 
height  in  all,  just  as  in  the  different  compartments  of  the 
vessel  represented  in  Fig.  139. 

A  man  once  thought  that  he  had  solved  the  chimerical  problem,  per- 
petual motion,  by  means  of  a  vessel  constructed  as  in  Fig.  141.  He  rea- 
soned in  this  way :  If  the  vessel  contain  a  pound 
of  water  and  the  tube  only  an  ounce,  since  an 
ounce  cannot  balance  a  pound,  the  water  in  the 
vessel  must  be  constantly  forcing  that  in  the  tube 
upward.  It  therefore  must  constantly  run  out  of 
the  outlet  of  the  tube,  and  as  it  flows  into  the 
vessel  the  circulation  must  go  on,  the  only  hin- 
drance to  perpetual  circulation  being  the  evap- 
oration of  the  water.  He  was  confounded  when 

he  discovered,  on  pouring  water  into  the  vessel,  that  it  stood  at  precisely 
the  same  level  in  the  vessel  and  in  the  tube.  He  forgot  that  a  common 
teapot  is  nearly  such  a  vessel,  and  yet  does  not  overflow. — A  glass  tube  on 
the  outside  of  a  cistern  or  a  boiler,  and  connected  with  it  at  the  bottom, 
shows  at  once  the  level  of  the  water  within. — Another  illustration  of  the 
fact  that  water  is  alwavs  seeking  its  level  is  found  in  the  water-pipes  which 


HYDROSTATICS. 


167 


distribute  water  to  the  inhabitants  of  large  cities :  the  water  pumped  into 
a  reservoir  situated  on  an  elevation  rises  by  the  action  of  gravity  and  its 
perfect  mobility  to  the  height  of  every  cistern  not  above  the  level  of  the 
reservoir,  no  matter  ho\v  much  lo\ver  may  be  the  depression  crossed  by  the 
pipes. — We  are  not  to  suppose  that  it  was  ignorance  of  this  law  of  liquids 
that  led  the  ancients  to  build  aqueducts  of  stone  at  immense  expense,  in 
some  cases  spanning  valleys  at  great  heights ;  but  this  enormous  labor 
was  necessitated  by  the  lack  of  a  suitable  material  such  as  iron. 

103.  Springs  and  Artesian  Wells. — The  principles  devel- 
oped in  the  previous  paragraphs  will  explain  the  phenom- 
ena of  springs,  common  wells,  and  Artesian  wells.  The 
crust  of  the  earth  is  largely  made  up  of  layers  of  different 
materials,  as  clay,  sand,  gravel,  limestone,  etc.  When  these 
were  formed  they  were  undoubtedly  horizontal,  but  they 
have  been  thrown  up  by  convulsions  of  nature  in  such 
a  way  that  they  present  every  variety  of  arrangement. 
Since  some  of  these  layers  are  much  more  pervious  to 
water  than  others,  the  rain  which  falls  and  sinks  into  the 
ground  often  makes  its  way  through  one  layer  lying  be- 
tween two  others  which  are  impervious  to  water,  and  so 
may  make  its  appearance  at  a  great  distance  from  the  place 
of  its  entrance,  and  at  a  very  different  height.  How  this 
explains  the  phenomena 
of  springs,  common 
wells,  and  Artesian 
wells  is  made  clear 
by  Fig.  142.  A  A  and 
EBB  are  designed  to 
represent  porous  layers 
of  earth  lying  between 
other  layers  which  are 
impervious  to  water.  Fig.  142. 

The  water  in  A  A  will  flow  out  at  C,  making  what  is  com- 
monly called  a  spring.  If  we  dig  a  well  at  F,  going  down 
to  the  porous  layer,  B  B  B,  the  water  will  rise  to  G,  be- 


168 


NATURAL   PHILOSOPHY. 


cause  this  is  on  a  level  with  the  surface  of  the  ground,  II. 

£3  /  / 

where  the  supply  of  water  enters.  From  this  point  it  may 
be  raised  by  a  pump.  If  the  well  be  dug  at  D,  the  water 
will  rise  not  only  to  the  surface,  but  to  E,  because  this  is 
on  a  level  with  II.  Water  is  sometimes  obtained  under 
such  circumstances  from  very  great  depths.  In  this  case 
the  porous  stratum  containing  the  water  is  reached  by 
boring,  and  then  we  have  what  is  termed  an  Artesian  well. 
The  name  comes  from  the  province  of  Artois,  in  France, 
where  this  operation  was  first  executed.  There  is  a  cele- 
brated well  of  this  sort  at  Grenelle,  a  suburb  of  Paris, 
where  the  water  rises  from  a  depth  of  nearly  1800  feet 
below  the  surface,  and  is  further  carried  to  a  height  of  a 
hundred  feet  above  it,  furnishing  a  supply  of  beautifully 
clear  water  at  the  rate  of  800,000  gallons  a  day. 

104.  Pressure  of  Liquids  is  in  Proportion  to  Depth. — The 
pressure  of  a  fluid  is  in  exact  proportion  to  its  depth.  For, 
all  the  particles  being  under  the  influence  of  gravity,  the 
upper  layer  of  them  must  be  supported  by  the  second,  and 
these  two  layers  together  by  the  third,  and  every  layer 
must  bear  the  weight  of  all  the  layers  above  it.  This 
pressure,  being  occasioned  by  the  weight  acting  vertically 
downward,  is  not  dependent  on  the  amount  of  the  sur- 
rounding liquid,  nor  on  the  shape  or  size  of  the  containing 
vessel.  In  a  vessel  having  the  shape  A,  Fig.  143,  it  is  evi- 


Pig.  143. 


H  YDKOSTATICS.  169 

dent  that  the  pressure  of  the  liquid  within  upon  the  bottom 
b  depends  upon  the  height  of  the  column  a  £;  in  the  ves- 
sel B,  which  widens  at  its  mouth,  the  pressure  on' the  bot- 
tom/is equal  to  that  of  the  weight  of  the  column  e  f, 
for  the  rest  of  the  liquid  exerts  pressure  upon  the  sides 
cf,df9  and  balances  the  column  e  f  on  all  sides  round 
about. 

In  the  case  of  a  vessel  tapering  at  its  mouth,  C,  let  us 
suppose  the  bottom  g  h  divided  into  a  number  of  portions 
of  the  same  size  as  the  mouth  L  and  that  there  are  ei<rht  of 

'  O 

these  portions.  The  pressure  on  the  bottom  g  h  is  the 
same  as  that  of  eight  columns  of  liquid  of  the  same  size  as 
i.  An  inspection  of  a 
Fig.  144  will  perhaps 
facilitate  comprehen- 
sion  of  this  law.  The 
pressure  on  the  bot- 
tom k  I  of  the  vessel  Fig.i44. 
D  is  equivalent  to  the  weight  on  the  liquid  column  m  n  Jc  I] 
the  pressure  on  g  i  depends  upon  the  height  of  column 
h  m  g  i,  etc.  In  the  case  of  a  vessel  having  the  form  E, 
the  bottom  op  has  to  bear  the  weight  of  the  liquid  column 
w  x  o  p\  this,  however,  has  itself  to  sustain  the  pressure 
of  the  column  u  v  s  t  exerted  upon  the  liquid  at  s  t,  and 
this  pressure  produces  an  effect  equivalent  to  that  of  a 
column  whose  base  is  o  p,  and  whose  height  is  o  q.  It  is 
evident  from  the  foregoing  that  the  total  pressure  of  a 
liquid  on  the  bottom  or  side  of  a  vessel  depends  on  the 
area  and  the  depth.  A  tube  two  feet  long  and  a  square 
inch  in  section  holds  nearly  a  pound  of  water;  hence  the 
pressure  of  water  at  any  depth,  whether  on  the  bottom  of 
a  vessel  or  on  its  side,  is  little  less  than  one  pound  on  the 
square  inch  for  every  two  feet  of  depth — a  general  truth 
worth  remembering. 


\ 


170  NATUKAL   PHILOSOrilY. 

Illustrations. — The  increase  of  pressure  at  great  depths  produces  the 
most  striking  effects.  Thus,  if  an  empty  corked  bottle  be  let  down  very 
deep  at  sea,  either  the  cork  will  be  driven  in  or  the  bottle  will  be  crushed 
before  it  reaches  a  depth  of  ten  fathoms.  A  gentleman  tried  the  follow- 
ing experiment :  He  made  a  pine-wood  cork,  so  shaped  that  it  projected 
over  the  mouth  all  around.  He  then  covered  this  with  pitch,  and  fastened 
over  the  whole  several  pieces  of  tarpaulin.  The  bottle,  thus  prepared,  he 
let  down  to  a  great  depth  by  attaching  to  it  a  weight.  On  raising  it  up 
he  found  that  it  contained  about  half  a  pint  of  water  strongly  impregnated 
•with  pitch,  showing  that  the  pressure  of  the  water  forced  water  through 
the  several  pieces  of  tarpaulin,  the  pitch,  and  the  pores  of  the  wooden  cork. 
When  a  ship  founders  near  land,  the  pieces  of  the  wreck,  as  it  breaks  up, 
float  to  the  shore  ;  but  when  the  accident  happens  in  deep  water,  the  great 
pressure  forces  water  into  the  pores  of  the  wood,  and  thus  makes  it  so  heavy 
that  no  part  of  the  vessel  can  ever  rise  again  to  reveal  her  fate.  When  a 
man  dives  very  deep,  he  suffers  ziuch  from  the  pressure  on  his  chest.  If 
we  watch  a  bubble  of  air  rising  in  water,  it  is  small  at  first,  but  it  grows 
larger  as  it  approaches  the  surface,  because  it  sustains  less  pressure  than 
when  deep  in  the  water.  The  force  with  which  a  fluid  is  discharged  from 
an  opening  in  a  vessel  depends  on  the  height  of  the  fluid  above  the  open- 
ing. The  difference  in  this  respect  between  a  full  barrel  and  one  nearly 
empty  is  very  obvious. — It  is  not  known  whether  there  is  a  limit  to  the 
pressure  which  fishes  can  bear  with  impunity,  but  they  abound  chiefly  in 
the  shallow  waters  on  coasts,  or  on  banks  in  the  midst  of  the  ocean,  such 
as  the  banks  of  Newfoundland,  or  the  bank  of  Lagullas,  off  the  Cape  of 
Good  Hope. 


105.  Sluice  -  Gates,  Dams,  etc. — The  application  of  the 
above  principles  in  the  construction  of  sluice-gates,  dams, 
etc.,  is  a  matter  of  great  practical  importance.  Pressure 
in  a  fluid  is  always  in  proportion  to  the  height  of  the  fluid 
above  the  point  of  pressure.  The  pressure  upon  any  por- 
tion of  the  side  of  a  vessel  containing  a  fluid  must  be  in 
proportion  to  its  distance  from  the  surface ;  or,  in  other 
•words,  it  is  the  weight  of  a  column  of  water  extending 
from  this  portion  to  the  surface.  Let  A  B  C  D  (Fig.  145) 
represent  a  section  of  a  cubical  vessel — that  is,  one  in  which 
each  side  is  of  the  same  size  with  the  bottom.  The  press- 


HYDROSTATICS. 


171 


ure  on  the  point  a,  in  the  line  A  B,  is  D 
that  of  a  column  of  particles,  A  a.  But 
A  a  is  equal  to  c  b,  and  c  b  is  equal  to 
b  a.  Therefore  b  a  may  represent  the 
pressure  on  a.  In  the  same  way,  it  can 
be  shown  that  e  d  represents  the  press- 
ure on  d,  n  in  the  pressure  on  m,  C  B  c 
that  on  B.  Therefore  the  pressure  on 
all  the  points  in  A  B  will  be  represented  by  lines  filling  up 
all  the  triangular  space  ABC,  and  this  is  half  of  A  B  C  D, 
which  represents  the  pressure  on  the  line  C  B.  It  is  clear, 
then,  that  since  the  pressure  on  a  vertical  line  in  the  side  is 
half  that  on  a  line  at  right  angles  to  it  in  the  bottom,  the 
pressure  on  the  whole  side  is  half  that  on  the  whole  bottom. 
We  see  from  the  above  demonstration  why  a  dam  is 
built  in  the  form  represented  in  Fig.  146. 
We  learn,  also,  why  the  hoops  and  other 
securities  at  the  lower  part  of  the  mon- 
strous vats  used  in  breweries  (some  of 
them  holding  many  thousand  barrels)  re- 
quire to  be  made  of  very  great  strength. 
It  is  manifest,  also,  that  if  a  sluice-gate  is  to  be  kept  shut 
by  a  single  support,  this  must  be  applied  at  one  third  of 
the  distance  from  the  bottom,  there  being  as  much  press- 
ure, as  shown  in  Fig.  145,  on  the  lower  third  as  on  the  up- 
per two  thirds  of  the  gate. 

106.  Lateral  Pressure  in  Fluids. — The  pressure  of  a  liquid 
on  the  side  of  a  vessel,  just  mentioned,  is  a  lateral  pressure, 
and  it  is  caused  by  the  downward  pressure  of  gravitation 
in  the  liquid.  But  how?  The  particles  of  a  fluid  are 
freely  movable  among  each  other,  and  therefore  are  ready 
to  escape  from  pressure  in  any  direction.  The  particles  at 
#,  Fig.  145,  pressed  upon  by  the  column  of  particles  ex- 
tending above  them  to  the  surface,  are  ready  to  escape 


172 


NATURAL   PHILOSOPHY. 


laterally,  and  would  do  so  if  an  opening  were  made  in  the 
vessel  at  that  point.  But  if  the  vessel  contained  a  block 
of  ice,  fitting  it  as  accurately  as  the  body  of  water,  there 
would  be  no  escape  at  the  opening,  because  the  particles 
of  the  solid  are  so  held  together  that  the  downward  press- 
ure of  the  earth's  attraction  occasions  no  lateral  pressure. 

The  manner  in  which  the  downward  pressure  of  the 
earth's  attraction  causes  lateral  pressure  may  be  made 
clear  by  Figs.  147  and  148.  We  will  sup- 
pose that  the  particles  of  solids  and  liquids 
are  alike  round,  and  that  a  solid  differs  from 
a  liquid  only  in  having  its  particles  firmly 
united  by  attraction.  Let  «,  #,  and  c,  in 
Fig.  147,  represent  three  particles  of  a  solid. 
Since  they  are  united  firmly,  they  will  have 
a  united  pressure  from  the  centre  of  gravity  directly  tow- 
ards the  centre  of  the  earth,  as  represented  by  the  arrow. 
Let  now  c7,  e,  and  J\  Fig.  148,  represent 
three  particles  of  water.  These  being  but 
very  slightly  coherent,  will  make  each  an 
independent  pressure  towards  the  earth's 
centre,  as  indicated  by  the  arrows.  It  is 
plain  that  d  tends  to  separate  e  and/*,  and 
will  do  so  if  they  are  left  free  to  move  in  a 
lateral  direction.  For  example,  if  e  be  at  the  side  of  a  ves- 
sel, and  an  opening  be  made  there,  the  downward  pressure 
.  of  d  will  give  e  a  lateral  movement, 

forcing  it  out  of  the  opening. 


Fig.  148. 


Fig.  141*. 


Another  View. — Referring  again  to  Fig.  145, 
here  repeated,  observe  that  the  lateral  pressure 
at  any  point  in  the  side  of  a  vessel,  as  a,  is  oc- 
casioned wholly  by  the  downward  pressure  of  a 
vertical  column  of  particles  extending  from  that 
point  to  the  surface.  The  neighboring  columns 


HYDROSTATICS.  173 

of  particles  have  nothing  to  do  with  it.  The  same  thing  is  true  in  regard 
to  any  other  point  either  in.  the  line  A  B  or  another  line  drawn  on  the 
side  of  the  vessel.  And  therefore  the  pressure  upon  the  whole  side  is 
occasioned  solely  by  the  columns  of  particles  in  close  proximity  to  it, 
and  not  at  all  by  the  other  columns  of  particles  in  the  vessel.  The  num- 
ber of  these  columns,  therefore,  in  the  vessel — or,  in  other  words,  tho 
breadth  of  the  body  of  water  in  it — makes  no  difference  in  the  pressure 
on  its  side.  For  this  reason  two  flood-gates  so  near  together  that  a  few 
hogsheads  or  even  pails  of  water  fill  up  the  space  between  them  sustain  as 
much  pressure  as  they  would  if  a  lake  or  an  ocean  of  water  lay  between 
them.  The  project  of  digging  a  ship-canal  between  the  Red  Sea  and  the 
Mediterranean,  now  so  triumphantly  accomplished,  was  objected  to  on  the 
ground  that  the  water  in  the  former  being  twenty  feet  higher  than  in  the 
latter,  it  would  burst  through  the  flood-gates  with  such  force  as  to  produce 
most  disastrous  results.  But  according  to  the  principle  just  illustrated, 
there  is  no  more  danger  of  this  than  there  would  be  if  two  ponds  were 
united  by  a  canal,  in  one  of  which  the  water  is  twenty  feet  higher  than  in 
the  other. 

107.  Pressure  in  Liquids  Equal  in  all  Directions. — We  are 
now  prepared  to  go  a  step  farther.  The  pressure  occasioned 
by  gravitation  in  fluids  operates  equally  in  all  directions 
when  the  fluid  is  at  rest.  That  is,  any  particle  of  a  liquid 
is  pressed  equally  in  all  directions.  If  it  were  not  so,  it 
would  not  remain  at  rest,  but  would  be  moved  in  the  di- 
rection in  which  the  superior  pressure  oper- 
ates. Suppose  that  a,  Fig.  150,  is  a  stratum 
of  particles  in  a  vessel  containing  water 
at  rest.  The  upward  pressure  on  it  being 
equal  to  the  downward  pressure,  the  stra- 
tum neither  rises  nor  falls.  If  a  body  of  liquid  be  disturbed 
by  wind  or  any  other  cause,  those  particles  which  are  raised 
above  the  common  level  in  waves  are  pressed  downward 
more  than  upward  or  laterally,  in  obedience  to  the  action 
of  gravitation.  They  therefore  move  downward,  pushing 
laterally  and  upward  the  neighboring  particles,  until  the 
liquid  regains  its  level  surface  and  its  state  of  rest.  In  like 

H 


174 


NATURAL   PHILOSOPHY. 


manner,  if  any  particles  are  heated  they  become  lighter 
than  their  neighboring  particles;  and  the  latter,  being  more 
strongly  attracted  than  the  former,  push  them. upward  in 
order  to  take  their  places.  When  all  the  liquid  acquires 
the  same  temperature,  it  is  at  rest,  each  particle  having  an 
equal  pressure  upon  it  in  all  directions. 

Illustrations. — If  a  bladder  filled  with  water  be  compressed  by  the  hand, 
the  water  is  pressed  no  more  immediately  under  the  hand  than  in  any 
other  part  of  the  bladder,  and  wherever  an  opening  be  made  the  water  will 
rush  out  with  equal  readiness.     A  hose-pipe  as  readily 
bursts  upward  as  in  any  other  direction.    A  large  cork, 
sunk  in  very  deep  water,  will  be  uniformly  reduced  in 
its  dimensions,  showing  that  it  has  been  pressed  equally 
on  all  sides.     In  the  experiments  with  the  closed  bot- 
tles (§  104),  the  result  is  the  same  if  the  bottle  be  sunk 
with  its  mouth  downward.     If  two  tubes,  shaped  as  in 
Fig.  151,  be  thrust  down  into  water,  the  water  will  rise 
with  equal  facility  in  both,  although  in  the  straight  one 
Fig.  151.  t|ie  pressure  which  carries  up  the  water  is  wholly  up- 

ward, while  in  the  bent  one  it  is  at  first  downward. 

^  108.  Upward  Pressure  as  the  Depth. — It  has  been  shown 
that  the  downward  and  the  lateral  pressure  are  in  propor- 
tion to  the  depth.  The  same  is  true  of  the  upward  pressure, 
and  owing  to  the  same  cause — the  attraction  of  the  earth. 
Let  us  examine  this  case.  Why  is  any  particle  of  a  fluid 
pressed  upward  at  all  ?  It  is  owing  to  the  struggle  on  the 
part  of  the  neighboring  particles  to  get  below  it.  And 
why  this  struggle?  It  results  from  the 
attraction  of  gravitation,  and  the  greater 
this  attraction  the  greater  the  upward 
pressure.  The  upward  pressure  there- 
fore, as  well  as  the  downward  pressure, 
differs  at  different  depths.  Thus,  in  Fig. 
152,  the  upward  pressure  against  the  layer 
or  stratum  of  particles,  #,  is  greater  than  Fig.  152. 


HYDROSTATICS. 


175 


153. 


that  against  a,  for  the  same  reason  that  the  downward 
pressure  on  b  is  greater  than  that  on  a.  But  the  two 
pressures  at  b  are  equal,  and  so  are  they  at  a,  and  there- 
fore each  stratum  remains  at  rest. 

Experiments.  —  Some  very  neat  experiments  show  that 
the  upward  pressure  varies  with  the  depth.  Take  a  large 
glass  tube,  A  BCD,  Fig.  153,  fitted  at  one 
end  with  a  circular  plate  of  brass,  which  may 
be  held  there  by  a  string,  F.  Thus  arranged, 
plunge  it  quite  deep  into  water,  and  you  will 
find  it  unnecessary  to  hold  on  to  the  string,  for 
the  brass  disk  will  be  held  tight  to  the  tube 
by  the  upward  pressure  of  the  water.  Now 
draw  up  the  tube  slowly,  and  at  length  the 
disk  will  fall  from  the  end  of  the  tube.  Why? 
Because  the  end  of  the  tube  has  come  to  a 
point  where  the  upward  pressure  of  the  water  is  less  than 
the  downward  pressure  of  the  disk.  To  succeed  with  this 
experiment,  the  end  of  the  tube  where  the  disk  is  applied 
must  be  very  even  and  smooth. 

Another  experiment  may  be  tried  in  this  way :  Tie  to  one  end  of  a  glass 
tube  a  piece  of  thin  India-rubber  or  of  a  bladder,  and  fill  the  tube  partly 
with  water.  The  India-rubber  will  of  course  bulge  out  or  become  convex 
from  the  weight  of  the  water.  Press  the  closed  end  down  a  little  way  in  a 
vessel  of  water,  so  that  the  level  in  the  tube  shall  be  above  the  level  in  the 
vessel.  The  India-rubber  is  still  somewhat  convex-,  for  the  upward  press- 
ure, being  in  proportion  to  its  distance  from  the  surface  of  the  water  outside 
of  the  tube,  is  not  so  great  as  the  downward  pressure  of  the  higher  water 
within  the  tube.  Lower  the  tube  so  that  the  level  in  the  tube  is  the  same 
with  that  in  the  vessel.  The  India-rubber  then  becomes  flat,  because  the 
downward  and  upward  pressures  upon  it  are  equal,  just  as  would  be  the 
case  with  a  stratum  of  water  in  the  same  place.  But  press  the  tube  lower 
down,  and  the  India-rubber  bulges  upward  into  the  tube,  because  the  up- 
ward pressure  is  then  greater  than  the  downward. 

109.  Illustrations  of  Liquid  Pressure. — You  are  now  pre- 


176  NATURAL   PHILOSOPHY. 

pared  to  understand  the  explanation  of  some  very  striking 
phenomena  in  the  pressure  of  liquids.  If  you  take  a  per- 
fectly tight  cask  filled  with  water,  and  screw  into  its  top  a 
long  tube,  you  can  burst  the  cask  by  pouring  water  into 
the  tube.  To  understand  this  you  must  bear  in  mind  two 
facts — that  the  fluid  in  the  cask  is  not  compressible,  and 
that  its  particles  move  freely  among  each  other. 
Any  pressure,  therefore,  exerted  upon  it  is  felt 
through  the  whole  of  it  equally.  "If  the  tube," 
says  Dr.  Arnott,  "  have  an  area  of  a  fortieth  of 
an  inch,  and  contain  when  filled  half  a  pound 
of  water,  that  water  would  produce  a  pressure 
of  half  a  pound  upon  every  fortieth  of  an  inch 
all  over  the  interior  of  the  cask,  or  of  nearly 
2000  pounds  on  every  square  foot — a  pressure 
greater  than  any  ordinary  cask  can  bear."  Sup- 
pose a  small  reservoir  of  water  exists  in  the  side 
of  a  mountain  wholly  closed  up,  and  that  water 
from  a  height  above  finds  its  way  to  it  by  a 
crevice,  it  may  by  its  pressure  even  burst  open 
Fig.  154.  the  ^e  Of  the  mountain.  And  it  matters  not 
how  large  or  small  the  crevice  may  be,  for  pressure  in  a 
liquid  is  only  as  the  height.  If  the  reservoir  be  ten  yards 
square  and  an  inch  deep,  and  the  fissure  leading  to  it  be 
but  an  inch  in  diameter  and  two  hundred  feet  in  height, 

^5  * 

it  is  calculated  that  the  pressure  of  the  water  in  the  fissure 
would  be  equal  in  force  to  the  weight  of  5000  tons. 

The  manner  in  which  these  effects  are  produced  may  be 
made  clear  by  Fig.  155.  Let  A  be  a  close  vessel  filled  with 
water,  and  let  a  tube,  b,  be  made  fast  in  it,  with  a  movable 
plug  or  piston  at  c.  If  the  surface  of  the  water  be  pressed 
upon  by  this  piston  with  the  force  of  a  pound,  the  water 
being  incompressible  and  its  particles  freely  movable  among 
each  other,  the  pressure  will  be  extended  equally  through 


HYDROSTATICS. 


177 


c 


all  the  water,  and  every 
portion  of  the  vessel  A 
of  equal  surface  with  c 
will  bear  a  pressure  to 
the  extent  of  one  pound. 
If  another  tube,  d,  of  the 
same  size  were  inserted 
with  a  piston,  i,  the  force 
of  a  pound  applied  to  the 
piston  c  would  push  upward  the  piston  i  with  the  same 
force.  And  if  there  were  several  pistons  of  the  same  size, 
by  pushing  upon  one  with  the  force  of  a  pound  they  would 
all  be  pressed  upward  with  exactly  this  force.  Further,  if 
e  be  a  tube  five  times  as  large  as  b,  its  piston,  n,  will  be 
forced  upward  with  a  pressure  of  five  pounds  by  the 
downward  pressure  of  a  pound  upon  c.  Suppose,  now, 
that  a  pound  of  water  were  substituted  for  the  piston  c, 
the  other  pistons  would  be  pressed  upward  as  before. 
And  if  all  the  pistons  be  removed,  the  pound  of  water  in 
b  will  press  the  water  up  the  tube  d  with  the  force  of  a 
pound,  and  up  the  tube  e  with  the  force  of  five  pounds. 

To  make  this  still  clearer,  we  will  present  it  in  a  little  different  form. 
Let  B,  Fig.  156,  be  a  close  vessel  with  two  tubes,  one  of  which  is  five 
times  as  large  as  the  other.  If  sufficient  water 
be  poured  into  the  vessel  to  occupy  a  part  of 
the  tubes,  it  will  stand  at  the  samo  height  in 
both  tubes,  as  indicated.  If  there  be  a  pound 
of  water,  then,  in  the  tube  c,  there  will  be  five 
pounds  in  a.  Now,  if  the  five  pounds  of  water 
in  a  pressed  any  heavier  on  the  whole  body  of 
water  in  B  than  the  pound  of  water  in  c,  it 
would  force  the  water  in  c  to  a  greater  height. 
But  this  is  impossible,  as  has  been  shown  in 

§  102.  Observe  that  the  pressure  of  five  pounds  in  a  is  spread  over  five 
times  the  area  or  extent  of  surface  as  the  pressure  of  one  pound  in  c.  If 
the  tube  c  have  an  area  of  an  inch  square,  the  water  in  it  will  exert  a 


178 


NATUKAL   PHILOSOPHY. 


pressure  of  a  pound  on  every  square  inch  in  the  vessel.  The  water  in  a 
exerts  a  pressure  of  live  pounds ;  but  it  must  be  remembered  that  it  does 
not  press  with  this  force  on  every  square  inch,  but  on  each  space  of  five 
square  inches,  and  that  therefore  its  pressure  on  each  inch  is  the  same  as 
that  in  the  tube  c. 

110.  Hydrostatic  Paradox. — It  is  evident  from  the  phe- 
nomena and  explanations  given  above  that  a  small  quantity 
of  a  fluid  can,  under  certain  circumstances,  exert  an  enor- 
mous pressure.  This  fact  has  been  called  the  Hydrostatic 
Paradox.  It  does  seem,  at  first  view,  incredible  or  para- 
doxical when  one  asserts  that  a  few  ounces  of  water  can 
be  made  to  raise  weights  of  hundreds  or  even  thousands  of 
pounds.  But  the  explanations  given  show  you  that  there 
is  no  unexplainable  mystery  in  the  fact.  The  cause  of  it 
is  the  same  as  that  which  gives  a  level  surface  to  liquids; 
viz.,  the  force  of  gravitation  acting  upon  a  substance  whose 
particles  are  freely  movable  among  each  other.  In  fact, 
there  is  nothing  more  paradoxical  in  it  than  that  one  pound 
at  the  long  end  of  a  lever  should  balance  ten  pounds  at  the 
short  end,  an  explanation  of  which  is  given  in  the  chapter 
on  the  Simple  Machines. 

Hydrostatic  .Bellows. — The  instrument  called  the  Hydro- 
static Bellows  is  represented  in  Fig.  157.  It  consists  of 
two  circular  boards,  A  and  B,  united  togeth- 
er by  strong  leather,  and  having  a  tube,  C, 
through  which  water  can  be  poured  into  it. 
The  weight  which  can  be  sustained  on  the 
bellows  without  forcing  the  water  out  of  the 
tube  depends  on  the  size  of  the  bellows.  If 
the  area  of  the  tube  be  only  one  thousandth 
of  that  of  the  top  of  the  bellows,  a  pound  of 
water  in  the  tube  will  balance  a  thousand 
pounds'  weight  on  the  bellows.  It  is  for  the 
same  reason  that  one  pound  of  water  in  the 


Fig.  15T. 


HYDROSTATICS. 


179 


tube  c  balances  five  pounds  in  a  (see  Fig.  156).  As  the 
weight  presses  upon  the  top  as  a  whole,  it  is  just  as  if  a 
vessel  of  the  same  size  with  the  bellows  were  resting  upon 
it  and  containing  a  thousand  pounds  of  water.  The  water, 
in  that  case,  would  stand  at  the  same  height  in  the  vessel 
and  in  the  tube.  This  shows  that  the  Hydrostatic  Paradox 
is  only  one  illustration  of  the  great  fact  that  a  liquid,  under 
the  influence  of  gravitation,  seeks  its  level. 

When  the  weight  on  the  bellows  is  less  than  is  required 
to  balance  the  water  in  the  tube,  the  weight  can  be  raised 
continually  by  pouring  water  into  the  tube.  But  observe 
that  although  the  lifting  force  be  so  strong,  it  is  very  slow 
in  its  operation.  If  the  comparative  areas  of  the  tube  and 

the  bellows  be  as  above  sup- 
posed, the  water  must  fall  in 
the  tube  ten  inches  in  raising 
the  weight  the  one  hundredth 
part  of  an  inch. 

111.  Bramah's  Hydrostatic 
Press. — These  principles  have 
been  applied  by  Mr.  Bramah 
in  his  Hydrostatic  Press. 


180  NATUKAL  PHILOSOPHY. 

This  consists  of  a  small  metallic  forcing-pump,  Fig.  158,  in 
which  the  water  in  the  reservoir  b  b  is  pumped  up  by  the 
piston,  Sj  worked  by  a  lever  not  shown  in  the  cut,  and 
forced  into  a  strong  and  large  cylinder,  c  c.  In  this  cylin- 
der is  a  stout  piston,  P,  having  a  flat  head,  n  n,  above. 
Between  this  plate  and  another,  e,  is  placed  the  body  to 
be  compressed.  It  is  obvious  that  the  pressure  exerted 
will  be  in  proportion  to  the  difference  between  the  size  of 
the  pump  and  the  cylinder,  c  c,  just  as  in  the  case  of  the 
bellows  it  depended  on  the  difference  between  the  areas 
of  the  tube  and  of  the  top  of  the  bellows.  "  If  the  pump 
have  only  the  one  thousandth  the  area  of  the  large  cylinder, 
and  if  a  man  by  means  of  its  lever-handle  press  its  piston 
down  with  a  force  of  one  hundred  pounds,  the  piston  of 
the  great  cylinder  will  rise  with  the  force  of  one  hundred 
thousand  pounds.  Scarcely  any  resistance  could  with- 
stand the  power  of 'such  a  press;  with  it  the  hand  of  a 
child  might  break  a  strong  iron  bar."  The  hydraulic  press 
is  of  great  service  in  the  mechanic  arts;  it  is  used  in  press- 
ing paper,  cotton,  hay,  and  other  bulky  yielding  substances, 
to  raise  great  weights,  to  test  the  strength  of  cables,  to 
launch  vessels,  to  force  the  oil  out  of  seeds,  and  for  many 
other  purposes. 


QUESTIONS. 

97.  What  constitutes  the  essential  difference  between  the  different  forms 
of  matter?  Give  the  classification  of  that  branch -of  Physics  relating  to 
fluids. — 98.  Why  do  liquids  at  rest  assume  a  level  surface?  Give  the  com- 
parison. Is  the  surface  of  a  liquid  strictly  horizontal  ?  Illustrate  this. 
Describe  the  spirit-level  and  its  uses. — 99.  What  is  said  of  the  flow  of 
rivers? — Illustrate  by  reference  to  the  Ganges  and  other  rivers. — 100.  How 
have  some  rivers  been  made?  What  is  stated  about  the  river  Danube? 
What  of  Lake  Geneva? — 101.  Describe  the  arrangement  of  canal  locks. 
Why  are  canals  so  useful  ? — 102.  Illustrate  the  tendency  of  liquids  to  rise 
to  the  same  level  by  reference  to  a  coffee-pot.  Describe  a  foolish  man's 


SPECIFIC   GRAVITY.  181 

plan  for  perpetual  motion,  and  give  the  reason  of  its  failure.  What  is  said 
of  ancient  and  modern  aqueducts? — 103.  Explain  the  operation  of  springs 
and  Artesian  wells.  Whence  comes  the  name  Artesian  ?  What  is  stated 
of  a  well  in  Paris? — 104.  Why  is  the  pressure  of  a  liquid  in  proportion  to 
its  depth  ?  Give  the  illustrations  of  this. — 105.  Explain  the  application  of 
these  principles  to  the  construction  of  dams.  What  is  said  about  the  con- 
struction of  dams  and  brewers'  vats? — 10G.  Explain  the  lateral  pressure 
of  liquids.  Show  the  difference  between  a  liquid  and  a  solid  in  this  re- 
spect. Show  how  the  earth's  attraction  causes  the  lateral  pressure.  Give 
another  view.  What  is  said  of  the  proposed  ship-canal  between  the  Medi- 
terranean and  the  Red  Sea? — 107.  Show  that  pressure  in  liquids  is  equal 
in  all  directions.  Give  the  illustrations.  — 108.  Show  that  the  upward 
pressure  in  a  liquid  is  as  the  depth,  and  that  this  is  produced  by  gravita- 
tion. Describe  the  experiment  represented  in  Fig.  152.  Give  the  experi- 
ment with  the  tube  and  India-rubber. — 109.  State  the  examples  given  of 
gre;it  effects  produced  by  small  quantities  of  a  fluid.  Explain  these  effects 
by  reference  to  the  diagram.  Explain  Fig.  15G. — 110.  What  is  the  Hy- 
drostatic Paradox,  and  why  is  it  so  called?  Describe  and  explain  the 
Hydrostatic  Bellows. — 111.  Describe  and  explain  Bramah's  Hydrostatic 
Press.  Mention  some  of  its  uses. 


CHAPTER  XL 

SPECIFIC    GRAVITY. 

112.  Nature  of  the  Subject. — We  now  reach  a  very  inter- 
esting subject,  intimately  connected  with  Hydrostatics,  and 
the  principles  which  have  been  developed  in  relation  to  liq- 
uids are  to  be  here  applied  to  various  kinds  of  substances. 
As  we  proceed  you  will  see  that  all  the  phenomena  brought 
to  view  in  this  chapter  are  to  be  referred  to  the  same  cause 
as  those  of  the  previous  chapter — viz.,  the  attraction  of 
gravitation. 

Before  proceeding  with  the  study,  we  will  define  Specific 
Gravity.  The  specific  gravity  of  any  substance  is  its  weight 

II  2 


182  NATURAL  PHILOSOPHY. 

as  compared  with  that  of  an  equal  volume  of  another  sub- 
stance taken  as  a  standard.  For  solids  and  liquids  distilled 
water  is  the  standard  of  comparison,  and  its  specific  gravity 
is  for  convenience  called  1.  Mercury,  for  example,  is  thir- 
teen and  a  half  times  as  heavy  as  an  equal  volume  of  water, 
and  is  said  to  have  a  specific  gravity  of  13.5.  For  gases  air 
is  usually  taken  as  the  standard,  though  hydrogen  gas  is 
sometimes  so  employed.  . 

We  shall  postpone  explaining  the  methods  of  determin- 
ing the  specific  gravities  of  bodies  until  we  have  more  fully 
detailed  the  principles  involved.  (See  §  116.) 

113.  Action  of  Gravity  on  Solids  in  a  Liquid. — The  reason 
that  a  very  heavy  substance — a  stone,  for  example — sinks  in 
water  is  simply  because  the  earth  attracts  it  more  strongly 
than  it  does  the  water,  and  drags  the  stone  down  through 
it.  If  the  stone  lay  upon  a  bladder  filled  with  water,  it 
would  press  upon  it  with  the  force  with  which  it  is  attract- 
ed by  the  earth.  But  where  water  is  not  thus  confined,  the 
stone  thrusts  its  particles  to  the  one  side  and  the  other  till 
it  gets  to  the  bottom. 

It  is  the  attraction  of  gravity,  also,  that  makes  light  sub- 
stances, as  wood  and  cork,  rise  in  water.  In  this  case  the 
water  is  attracted  by  the  earth  more  strongly  than  the 
wood  or  cork,  and  so  gets  below  it,  and  in  so  doing  pushes 
up  the  lighter  substance. 

But  you  will  observe  that  the  wood,  on  rising  in  the  wa- 
ter, does  not  come  completely  out  of  it  and  lie  upon  the 
surface,  but  a  part  of  it  remains  im- 
mersed in  the  water.  The  explanation 
of  this  will  furnish  you  with  the  key  to 
many  very  interesting  facts.  Suppose 
that  half  a  block  of  wood,  A,  Fig.  159, 
weighing  a  pound,  is  above  the  surface 


ig.159.  o^  water<      ^g  jt  js  attracted  to  the 


SPECIFIC   GRAVITY. 


183 


earth  with  the  force  of  a  pound,  it  has  pushed  to  the  one 
side  and  the  other  just  a  pound  of  water,  and  taken  its 
place.  It  is  drawn  down  towards  the  earth  with  the  same 
force  as  the  pound  of  water  on  either  side  of  it,  b  or  c. 
If  it  were  attracted  more  strongly — that  is,  if  it  weighed 
more  than  a  pound — it  would  displace  more  than  a  pound 
of  water.  If  it  had  just  the  same  weight  as  the  same  vol- 
ume of  water,  it  would  displace  a  volume  of  water  equal 
to  its  own  bulk ;  it  would  be  wholly  immersed,  and  would 
stay  in  the  water  wherever  you  placed  it,  because  it  is  at- 
tracted by  the  earth  with  the  same  force  as  an  equal  bulk 
of  water. 

Imagine  water  in  a  vessel  divided  into  equal  portions  of 
a  pound  each,  as  represented  in  Fig.  160.  Now,  suppose  that 
the  portion  a  should  at  once  change  into 
solid  ice  without  at  all  altering  its  bulk  or 
weight.  It  would  not  move  from  its  posi- 
tion, because  it  is  attracted  by  the  earth 
with  precisely  the  same  force  as  when  it 
was  water,  and  as  strongly  as  each  of  the 
equal  portions  of  water  around  it.  But  Fig.ico. 
since  water  on -becoming  ice  does  really  increase  in  bulk 
and  therefore  become  lighter,  this  block  of  ice  would  rise 
so  that  a  part  of  it  would  be  above  the  surface. 

The  lighter  a  substance  immersed  in  water,  the  more 
of  it  will  there  be  above  the  surface.  Consider  the  case 
of  two  blocks  of  wood  having  different  weights,  though 
of  the  same  size.  Suppose  the  heavier  one,  A,  Fig.  161,  is 
one  third  lighter  than  the  same  bulk 
of  water.  One  third  of  it  will  be 
above  the  surface.  If  the  other,  B, 
be  half  the  weight  of  water,  half  of  it 
will  be  above  the  surface.  We  would 
say,  then,  that  the  specific  gravity  of 


Fig.  161. 


184  NATURAL  PHILOSOPHY. 

the  wood  in  the  first  block  is  two  thirds  that  of  water,  and 
the  specific  gravity  of  the  wood  in  the  second  is  one  half 
that  of  water. 

Illustrations. — There  are  many  interesting  facts  illustrating  the  princi- 
ples here  developed.  A  stone  is  lifted  much  more  easily  in  water  than  in 
air  because  of  the  support  afforded  by  the  upward  pressure  of  the  water. 
A  boy  often  wonders  why  he  can  lift  a  very  heavy  stone  to  the  surface,  but 
can  raise  it  no  farther.  When  a  bucket  of  water  is  drawn  up  a  well,  much 
less  exertion  is  required  to  raise  it  through  the  water  than  through  the  air 
after  it  emerges  from  the  water.  While  it  is  in  the  water  you  raise  only 
the  bucket  itself;  the  water  in  it  exerts  no  pressure  on  the  rope,  being  sus- 
tained by  the  water  around  it.  But  when  it  reaches  the  air,  you  have  to 
raise  the  weight  of  the  water  added  to  that  of  the  bucket.  When  a  person 
lies  in  a  bath  for  some  time,  on  raising  his  arm  from  the  water  it  seems  to 
be  very  heavy.  This  is  because  the  arm  has  had  for  so  long  a  time  the 
support  of  the  water  that  when  it  is  lifted  into  the  air  the  want  of  this  sup- 
port is  sensibly  felt.  It  is  said  that  Archimedes  conceived  of  the  princi- 
ples of  specific  gravity  as  his  limbs  felt  the  liquid  support  of  a  bath,  and  so 
overjoyed  was  he  with  the  discovery  that  he  ran  home,  crying  out  all  the 
way,  "Eupjjcca!  fvprjKal" — I  have  found  it!  I  have  found  it!  It  was  a 
rational  joy,  for  he  had  found  a  principle  of  immense  value  to  science  and 
to  the  world. 

Boats  and  Life-boats. — A  boat  of  iron  will  float  as  high  out  of  water 
as  one  of  wood  of  the  same  size,  provided  the  iron  be  made  so  thin  that 
the  boat  is  not  heavier  than  the  wooden  one.  For  what  is  it  that  floats  ? 
Not  the  iron  or  wood,  but  a  wooden  or  iron  boat  filled  with  air.  If  it  were 
filled  with  water  instead  of  air,  it  would  sink,  the  specific  gravity  of  the 
materials  of  which  it  is  built  being,  on  the  whole,  of  greater  specific  gravity 
than  water.  Life-boats  have  in  their  structure  either  a  large  quantity  of 
cork  or  air-tight  vessels  of  tin  or  copper,  and  consequently  are  so  light 
that  they  will  float  even  when  filled  with  water. 

114.  Specific  Gravity  of  Animals. — Birds  have  a  much 
less  specific  gravity  than  animals  that  walk,  in  order  that 
they  may  rise  easily  in  the  air.  Their  light  feathers  in- 
crease greatly  their  bulk,  as  you  may  see  whenever  a  bird 
is  stripped  of  them.  Besides  this,  the  bones  are  hollow  and 
communicate  with  the  lungs.  Birds  that  swim,  as  ducks, 


SPECIFIC    GRAVITY.  185 

swans,  etc.,  have  so  small  a  specific  gravity — that  is,  are  so 
large  in  proportion  to  their  weight — that  but  a  small  part 
of  the  body  is  under  water,  and  the  motion  of  their  feet  is 
not  required  to  sustain  them,  but  serves,  like  oars,  to  pro- 
pel them  along.  Insects  are  of  small  specific  gravity, 
those  that  fly  the  most  swiftly  being  the  lightest.  Fishes 
have  very  nearly  the  same  specific  gravity  as  water,  and 
hence  require  but  little  muscular  effort  to  move  about  in 
their  element.  They  are  assisted  much  in  rising  and  fall- 
ing by  a  contrivance  by  which  they  can  instantaneously 
alter  their  specific  gravity.  They  have  an  air-bladder, 
which  they  can  dilate  or  contract  at  pleasure.  When 
dilated,  the  bulk  of  the  fish  is  increased  and  his  specific 
gravity  lessened,  and  he  rises  easily  and  at  once.  By  com- 
pressing it  he  as  readily  sinks. 

The  human  body,  when  the  chest  is  filled  with  air,  is  so 
much  lighter  than  water  that  it  will  float  with  about  half 
the  head  above  the  surface.  A  knowledge  of  this  fact,  with 
proper  presence  of  mind,  might  ordinarily  save  persons 
from  drowning ;  for  if  the  body  be  put  in  the  proper  posi- 
tion, the  feet  downward  and  the  head  thrown  backward, 
the  nose  and  mouth  will  be  out  of  the  water.  So  little  is 
required  in  the  way  of  support  to  keep  the  whole  head  out 
of  water,  that  persons  who  cannot  swim  are  often  saved 
from  drowning  by  grasping  very  sn\all  pieces  of  wood. 
An  oar  would  support  half  a  dozen  men  if  they  would  be 
satisfied  with  keeping  only  the  head  out  of  water;  but  if 
each  one  struggle  to  get  his  whole  body  upon  the  oar,  they 
may  all  be  lost. 

'MIS.  Avoidable  Causes  of  Drowning. — The  reasons  that 
in  water-accidents  so  many  people  are  drowned  who  might 
easily  be  saved  are  thus  summarized  by  Dr.  Arnott : 

1  st.  They  believe  that  the  body  is  heavier  than  water ;  and,  therefore, 
that  unless  continued  exertion  ba  made,  they  must  sink.  Hence,  instead 


186  NATURAL  PHILOSOPHY. 

of  lying  quietly  and  a  little  on  the  back,  with  the  face  only  out  of  the  wa- 
ter, they  generally  assume  the  position  of  a  swimmer,  in  which  the  face  is 
downward,  and  the  whole  head  has  to  be  kept  out  of  the  water  to  allow 
of  breathing.  To  do  this  requires  practice ;  and  if  a  person  cannot  swim, 
the  first  attempt  at  floating  in  this  position  will  prove  a  disastrous  failure. 

2d.  The  body  raised  for  a  moment  by  any  exertion  above  the  floating 
level  sinks  as  far  below  that  when  the  exertion  ceases,  and  the  plunge  terri- 
fies the  unpractised  and  renders  them  easier  victims  to  their  fate. 

3d.  They  make  a  wasteful  exertion  of  strength  to  prevent  water  enter- 
ing the  ears,  not  thinking  that  it  can  only  fill  the  outer  ear,  as  far  as  the 
drum,  and  that  this  is  of  no  consequence. 

4th.  They  generally  attempt,  in  their  struggle,  to  keep  their  hands  free 
above  the  surface,  forgetting  that  any  part  of  the  body  held  out  of  the  wa- 
ter in  addition  to  the  face  (which  must  be  out)  requires  an  additional  effort 
to  support  it.  The  tendency  of  the  body  to  sink  diminishes  just  in  propor- 
tion to  the  quantity  immersed ;  because  all  those  parts  which  are  out  of 
water,  not  being  supported  by  the  water,  become  so  much  additional  abso- 
lute weight  to  the  portion  immersed.  This  is,  indeed,  one  of  the  most  fre- 
quent causes  of  death  by  drowning. 

5th.  If  the  accident  occur  at  sea,  they  cannot,  like  the  practised  swim- 
mer, choose  the  proper  interval  for  breathing,  which  is  when  the  crest  of  a 
wave  has  passed  over,  and  the  head  is  for  an  instant  above  water. 

Gth.  The  chest  should  be  kept  as  full  of  air  as  possible,  which  without 
other  effort  will  cause  nearly  the  whole  head  to  remain  above  water.  If  the 
chest  be  once  emptied  while  the  face  is  under  water,  and  the  person  cannot 
inhale  again,  the  body  remains  specifically  heavier  than  water,  and  will  sink. 

A  life-preserver  is  a  great  aid  in  preservation  from 
drowning,  for  it  diminishes  the  specific  gravity  of  the  body. 
It  is  commonly  an  air-tight  bag  fastened  round  the  upper 
part  of  the  body,  which  can  be  filled  when  required  by 
blowing  into  it  through  a  tube  fitted  with  a  valve.  "  On 
the  great  rivers  of  China,"  says  Dr.  Arnott,  "  where  thou- 
sands of  people  find  it  more  convenient  to  live  in  covered 
boats  upon  the  water  than  in  houses  on  the  shore,  the 
younger  male  children  have  a  hollow  ball  of  some  light 
material  attached  constantly  to  their  necks,  so  that  in  their 
frequent  falls  overboard  they  are  not  in  danger." 


SPECIFIC    GRAVITY.  187 

In  wading  a  river  the  feet  press  upon  the  bottom  with  a 
force  equal  to  half  the  weight  of  the  person's  head,  this  be- 
ing the  difference  between  the  weight  of  the  body  and  the 
weight  of  the  same  bulk  of  water.  Now,  this  pressure  is 
not  sufficient  to  give  a  sure  footing  against  even  a  moderate 
current.  Many  persons  have  been  drowned  from  ignorance 
of  this  fact.  A  man  carrying  a  load  may  often  ford  a  river 
vsafely  where  without  a  load  to  press  him  down,  and  thus  give 
him  a  sure  footing,  he  would  be  carried  down  the  stream. 

116.  How  to  Determine  the  Specific  Gravity  of  Solids. — 
From  the  principles  explained  in  the  preceding  pages  it  is 
evident  that,  owing  to  the  upward  pressure  of  water,  a  body 
weighs  less  in  water  than  in  air;  hence  we  determine  its 
specific  gravity  by  comparing  its  weight  in  water  with  its 
weight  in  air — water,  you  remember,  being  the  standard. 

In  determining  the  specific  gravity  of  solid  bodies,  sev- 
eral cases  may  arise :  the  solid  may  be  (I.)  heavier  than  wa- 
ter and  insoluble  in  it,  (II.)  heavier  than  water  and  soluble 
in  it,  (III.)  lighter  than  water  and  insoluble  in  it,  and  (IV.) 
lighter  than  water  and  soluble  in  it.  For  examples  of  the 
first  case  we  have  gold,  lead,  and  the  other  heavy  metals, 
also  rocks  and  minerals  of  various  kinds;  as  an  example  of 
the  second  case  we  have  sugar,  salt,  saltpetre,  and  many 
other  substances ;  cork,  the  varieties  of  wood,  and  the  bod- 
ies of  animals  are  examples 
of  the  third  case ;  the  fourth 
case  is  very  rare. 

(I.)  We  will  consider  the 
simplest  first,  and  take  for  an 
example  a  piece  of  lead.  Sus- 
pend the  lead  by  means  of  a 
hair  from  one  of  the  pans  of  a 
balance,  as  shown  in  Fig.  162, 
and  weigh  it  carefully.  Then  Fig.  102. 


188  NATURAL   PHILOSOPHY. 

introduce  the  lead  into  a  cup  of  water,  and  you  will  find 
that  a  portion  of  the  weight  must  be  removed  from  the 
opposite  pan  to  preserve  the  equilibrium.  The  weight 
which  you  take  from  the  pan  will  be  the  weight  of  a 
quantity  of  water  equal  in  bulk  to  the  piece  of  lead,  for 
the  immersed  body  is  supported  with  a  force  equal  to  the 
weight  of  the  water  it  displaces  (§  113).  Thus  if  a  piece 
of  lead  weighing  eleven  grammes  weigh  only  ten  grammes 
in  water,  it  will  prove  that  lead  is  eleven  times  as  heavy 
as  water.  And  if  a  lump  of  copper  weigh  nine  grammes 
in  air  and  eight  in  water,  it  is  nine  times  as  heavy  as  wa- 
ter. Calling,  therefore,  water  =1,  the  specific  gravity  of 
lead  is  11  and  of  copper  9.  It  is  obvious  that  a  body  of 
the  same  specific  gravity  Avith  water  would  weigh  nothing 
when  immersed  in  water,  for  it  would  be  supported  with 
an  upward  pressure  precisely  equal  to  its  own  weight,  just 
as  the  same  bulk  of  water  is.  A  hundred  grammes  of  wa- 
ter, therefore,  will  weigh  nothing  in  water.  The  experiment 
can  easily  be  tried.  Weigh  an  empty  glass  bottle,  sus- 
pended from  one  arm  of  the  scale-beam,  and  then  put  a 
hundred  grammes  of  water  in  it.  On  immersing  it  in  wa- 
ter it  will  be  balanced. 

Archimedes  and  the  Crown. — Hiero,  King  of  Syracuse,* 
stipulated  for  a  crown  of  pure  gold.  But  suspecting  the 
maker  of  it  had  adulterated  the  gold,  he  called  upon  Archi- 
medes to  detect  the  imposture.  He  did  it  in  this  way: 
he  procured  two  lumps  of  gold  and  silver  of  the  same 
weight  with  the  crown,  and  observed  the  quantity  of  water 
which  each  displaced.  He  then  tried  the  crown,  and  found 
that  it  displaced  less  than  the  silver  and  more  than  the 
gold,  and  therefore  concluded  that  it  was  an  alloy  of  the 
two  metals.  All  this  was  suggested  to  him  by  his  experi- 
ence in  the  bath,  referred  to  in  §  113. 

*  Hiero  II. ,  died  21 G  B.C. 


SPECIFIC   GRAVITY.  189 

117.  More  about  Determining  Specific  Gravity. — The  process 
explained  in  §  116  is  called  the  method  by  direct  weighing.  We  may  con- 
dense the  instructions  into  the  following  rule :  To  find  the  specific  gravity 
of  a  solid  heavier  than  water  and  insoluble  in  it,  weigh  it  in  the  air,  and 
again  in  water ;  then  divide  the  first  weight  by  the  difference  between  the 
first  and  the  second  weight.  Suppose  we  represent  the  weight  of  the 
solid  by  the  letter  W,  its  weight  in  water  by  Z,  and  the  words  specific 
gravity  by  the  abbreviation  sp.  gr.,  then  we  may  write  the  rule  in  the  fol- 
lowing manner : 


Observe  that  W—  Z  gives  us  the  weight  of  the  water  displaced.  An  exam- 
ination of  the  following  proportion  will  show  how  the  above  equation  is 
obtained : 

5       Weight  of       V       (    Sp.  gr.    \         I    Weight  of  ^         (  Sp.  gr.  \ 

water  displaced,  I         ]  of  water,  (...)  body  in  air,  (        ]      of      ! 

or               (    :     )         or         (   "    )            or  (     '    ]      the      j 

W-Z                    I        1        J                    W  I  body.   J 

1  x  W        W 
Whence,    Sp.  8l''=yV  —  7=w  —  7 

Example.        A  piece  of  lead  weighs  in  the  air  8.19  grammes. 

"  "         "      in  water    7.47         " 

Difference  (or  W-Z),  .72        " 

.72:1  ::  8.19:  sp.  gr. 

gp.  gVm=-L-— — =11.37,  sp.  gr.  of  lead. 
.  t  2 

(I.  A.)  Another  way  of  ascertaining  the  specific  gravity 
of  a  body  heavier  than  water  and 
insoluble  in  it  is  known  as  the  meth- 
od by  the  flask;  it  is  particularly 
applicable  to  fragments  of  minerals 
or  substances  in  powder.  For  this 
method  small  bottles  of  peculiar  con- 
struction are  used;  they  are  pro- 
vided with  a  stopper  ground  to  fit 
the  neck  of  the  bottle  well,  and 
pierced  by  a  small  hole  running  ver-  Fig.  163. 


190  NATUKAL  PHILOSOPHY. 

tically  through  it.  When  the  flask  is  filled  with  water 
aud  the  stopper  inserted,  that  portion  in  the  neck  escapes 
through  the  stopper,  permitting  the  flask  to  be  completely 
filled  with  precisely  the  same  weight  of  liquid  each  time  it 
is  used. 

Suppose  you  want  to  ascertain  the  specific  gravity  of  a  certain  kind  of 
sand :  weigh  a  portion  first  in  the  air ;  then  weigh  the  flask  filled  with 
water ;  next  introduce  the  sand  into  the  flask,  and  allow  it  to  force  out 
the  water  (equal  in  bulk  to  the  solid) ;  insert  the  stopper  carefully,  wipe 
the  flask  dry,  and  weigh  it  again.  The  specific  gravity  may  then  be  cal- 
culated from  the  following  equation  : 
Let  weight  of  sand  = W 


of  flask  and  water =W 
Let  weight  of  flask,  water,  and 
sand  =W" 


Then, 


Sp.  gr.  = 


W 


(II.)  To  determine  the  specific  gravity  of  a  substance  soluble  in  water> 
a  known  weight  of  it  is  weighed  in  oil  or  some  liquid  which  does  not  dis- 
solve it,  and  the  specific  gravity  of  the  oil  having  been  determined  by  some 
one  of  the  methods  explained  in  §  118,  the  specific  gravity  of  the  substance 
is  calculated  by  means  of  the  following  formula : 

If  the  weight  of  the  substance  in  air=W 
and        "  "  "        inoil=W 

Sp.  gr.  of  the  oil  =A 

Sp.  gr.  of  water  being  =  1 

then  W— W'=W"=the  liquid  displaced  ; 
and  A:1=W":W" 

W 

Whence  sp.  gr.  of  substance  — T^rrr 

(III.)  To  determine  the  specific  gravity  of  a  body  lighter  than  water 
and  insoluble  in  it,  weigh  it  first  in  the  air,  then  attach  to  it  a  piece  of  lead 
sufficiently  heavy  to  sink  it,  and  weigh  the  two  together  in  water ;  lastly, 
weigh  the  lead  alone  in  water,  then  calculate  from  this  formula : 
Weight  of  cork  in  air  =  W 


"         lead  in  water  =W 

Weight  of  lead  and  cork  in 

water  -W" 


Then, 

Sp-gr.  ==^77^ 


W'-W"+W 


SPECIFIC    GRAVITY.  191 

(IV.)  The  fourth  case,  that  of  substances  lighter  than  water  and  soluble 
in  it,  is  of  comparatively  rare  occurrence  ;  examples  are  found,  however, 
in  the  case  of  the  alkaline  metals,  sodium,  potassium,  etc.  Weigh  the  body 
in  the  air,  then  in  some  liquid  of  low  specific  gravity  in  which  the  body  is 
not  soluble—  naphtha,  for  example—  and  calculate  as  below  : 

Weight  of  body  in  air          =  W 
"  "          naphtha=W 

\v-w'=w" 

Sp.  gr.  of  naphtha  =  A 

"      water  =1 

A:  W"::l  :  W'" 


Before  proceeding  to  the  determination  of  the  specific  gravity  of  liquids, 
we  will  give  one  more  formula  which  enables  us  to  find  the  weight  of  each 
of  two  substances  when  combined  in  one  mass.  Some  such  formula  must 
have  been  used  by  Archimedes  in  ascertaining  the  proportion  by  weight  of 
the  gold  and  silver  forming  the  alloy  of  which  Hiero's  crown  was  made 
(§  H6). 

Sp.  gr.  of  the  alloy  =Sp.  gr. 

Weight     "     alloy  =W 

Sp.  gr.  of  one  constituent    —  s' 

Sp.  gr.  of  second      "  =  s" 

Weight  of  one          "  =w' 

Weight  of  second    "  -  w" 

Then, 


(s  -s  )  sp.  gr. 
And  w//=w_w,* 

To  insure  accuracy,  all  determinations  of  specific  gravity  should  be  made 
at  one  and  the  same  standard  temperature.  This  fixed  temperature  is  4°  C. 
—  that  at  which  water  has  its  greatest  density. 

118.  Determination  of  the  Specific  Gravity  of  Liquids.  — 
Several  methods  may  be  employed  for  ascertaining  the 

*  For  proofs  of  this  formula  see  "  Galloway's  First  Step  in  Chemistry," 
p.  74. 


192  NATURAL   PHILOSOPHY. 

specific  gravities  of  different  liquids.  We  will  describe 
three  of  them:  I.  By  the  flask;  II.  By  weighing  a  sub- 
stance in  it  ;  III.  By  the  hydrometer. 

I.  The  method  by  the  flask  is  exceedingly  simple.  Hav- 
ing selected  a  specific-gravity  flask,  determine  its  weight, 
fill  it  with  water,  and  weigh  again.  Then  empty  it,  dry  it 
carefully,  and,  filling  it  with  the  liquid  of  which  the  specific 
gravity  is  desired,  weigh  again. 

Let  weight  of  flask  =F 

"        "  "     and  water  =W 

"        "  "      "    liquid=W 

W'—  F 


Then,  Sp.  gr.  of  liquid  — 


TTT  —  r; 


II.  Take  a  body  of  known  specific  gravity,  and  insoluble 
in  the  liquid  to  be  examined ;  weigh  the  body  in  air,  and 
then  in  the  liquid ;  if 

Weight  of  body  =  W 

"        in  liquid =W 
Sp.  gr.  of  body  ==A 

W:(W-WO::A:Sp.  gr. 

*,*»&$& 

III.  The  most  expeditious  method  of  ascertaining  the 
specific  gravity  of  liquids  is  by  means  of  an  instrument 
called  a  hydrometer.     This  instrument  consists  of  a  glass 
tube  widened  into  a  large  and  a  small  bulb  at  one  end,  the 
smaller  bulb  containing  a  few  shot  or  a  little  mercury  to 
cause  the  centre  of  gravity  of  the  instrument  to  fall  in  the 
lower  part;   the  narrow  portion   of  the  tube,  called  the 
stem,  is  furnished  with  a  scale  for  reading  the  depth  to 
which  the  instrument  sinks  when  plunged  in  any  liquid. 
The  lighter  the  liquid  to  be  tested,  the  deeper  will  the 
hydrometer  sink  in  it.     The  manner  of  using  a  hydrometer 
is  obvious:  it  is  simply  floated  in  the  liquid  to  be  tested, 


SPECIFIC   GEAVITY.  193 

and  the  figure  on  the  scale  at  the  point  where  it 
touches  the  upper  surface  of  the  liquid  is  accurate- 
ly noted.  The  graduation  of  the  hydrometer  varies 
for  each  liquid ;  or,  if  the  scale  indicates  specific 
gravity,  and  not  arbitrary  degrees,  the  relation 
between  specific  gravity  and  the  strength  of  the 
liquid  examined  is  ascertained  by  reference  to  ta- 
bles printed  for  the  purpose.  Hydrometers  receive 
different  names  according  to  the  liquids  for  which 
they  are  constructed:  that  for  testing  alcohol  is 
called  an  alcoholometer /  for  solutions  of  sugar,  sac- 
charometer  ;  for  milk,  a  lactometer. 

Both  in  Europe  and  America  the  lactometer  is 
used  to  test  the  quality  of  milk.  In  large  cities 
the  adulteration  of  milk  with  water  has  become  so 
common  a  fraud  that  the  police  (in  some  instances) 
are  authorized  to  collect  samples  of  suspected  milk 
for  examination  with  the  lactometer.  In  New  York 
City  the  Board  of  Health  has  recommended  a  cer- 
tain lactometer  as  a  standard.  Besides  the  forms 
of  hydrometer  mentioned,  there  are  many  others;  p.  1M 
as,  for  example,  the  salimeter  (for  salt  solutions), 
the  vinometer  (for  wines),  the  acidometer  (for  acids),  etc. 
In  determining  the  specific  gravity  of  liquids,  attention 
must  be  paid  to  the  temperature  at  which  the  observa- 
tion is  made,  for  bodies  increase  in  volume  with  a  rise 
of  temperature,  and  this  increase  is  not  uniform  for  all 
substances. 

119.  Tables  of  Specific  Gravity. — Use  is  made  of  a  knowl- 
edge of  the  specific  gravity  of  certain  substances  to  identify 
them ;  especially  is  this  the  case  with  precious  stones  and 
minerals.  We  give  below  two  tables — one  of  the  specific 
gravity  of  solids,  and  the  other  of  liquids.  Observe  that 
the  specific  gravity  of  living  men  being  0.89,  or  lighter 


194 


KATUEAL  PHILOSOPHY. 


than  water,  they  should  float  if  the  precautions  mentioned 
in  §  114  were  properly  taken. 

TABLES   OF    SPECIFIC   GRAVITY. 


Solids. 

Cork 0.24 

Oak-wood 0.84 

Living  men 0.89 

Starch 1.50 

Alum 1.70 

Charcoal 1.85 

Roll  sulphur 2.00 

Saltpetre 2.10 

Quartz 2.G5 

Marble 2.83 

Glass  (flint) 3.33 

Diamond 3.52 

Iron  pyrites 5.00 

Tin 7.29 

Iron  (malleable) 7. 84 

Copper  (cast) 8. 78 

Silver  (fused) 10.50 

Lead 11.34 

Gold 19.50 

Platinum..               .  21.50 


Liquids. 

•Gasoline 0.66 

"B."  Naphtha 0.72 

Ether 0.72 

Kerosene  oil 0. 80 

Alcohol  (absolute). . .  0. 80 

Oil  of  turpentine 0. 86 

Ammonia  (solution) .  0.87 

Olive-oil 0.92 

Distilled  water 1.00 

Sea  water 1.02 

Milk  (cow) 1.03 

Human  blood 1.06 

Water  of  Dead  Sea..  1.16 

Glycerin 1.27 

Chloroform 1.49 

Nitric  acid 1.51 

Sulphuric  acid 1.84 

Bromine 2.98 

Thallium  ethylate. . .  3.55 

Mercury 13. 59 


Specific  Gravity  of  Gases. — The  specific  gravity  of  gases 
is  determined  by  a  process  much  like  that  for  liquids,  men- 
tioned in  §  118.  Air  (sometimes  hydrogen)  is  assumed 
as  the  standard.  A  large  glass  globe  filled  with  air  is 
weighed,  then  exhausted  by  an  air-pump,  and  weighed 
again,  the  access  of  air  being  prevented  by  a  stop -cock. 
The  difference  between  the  weights  gives  the  weight,  A, 
of  a  certain  volume  of  air;  the  globe  is  then  filled  with 
the  gas  under  examination,  and  weighed  a  third  time ;  by 
subtracting  the  weight  of  the  empty  globe  the  weight, 

T> 

B,  of  the  gas  is  obtained.     And  — ,  or  the  weight  of  the 

A 

gas   divided  by  the  weight  of  an  equal  volume   of  air, 


SPECIFIC   GKAVITY.  195 

gives  the  specific  gravity  of  the  gas.     Corrections  must 
of  course  be  made  for  temperature. 


QUESTIONS. 

112.  Explain  what  is  meant  by  specific  gravity.  What  are  the  stand- 
ards of  comparison  for  the  three  forms  of  matter? — 113.  Explain  the  sink- 
ing of  heavy  substances  in  water.  Explain  diagrams  Figs.  159  and  160. 
Give  the  illustrations  :  lifting  a  stone  ;  raising  a  bucket ;  raising  the  arm  in 
a  bath.  Relate  the  anecdote  of  Archimedes.  What  is  said  of  iron  boats  ? 
— 114.  What  is  said  of  the  specific  gravity  of  birds?  Of  insects?  Of 
fishes  ?  What  of  the  specific  gravity  of  the  human  body  ? — 115.  State  the 
principal  avoidable  causes  of  drowning.  What  is  narrated  about  children 
in  China?  Why  is  wading  in  deep  rivers  sometimes  dangerous? — 116. 
What  four  cases  may  arise  in  determining  the  specific  gravity  of  sub- 
stances ?  Explain  the  manner  in  which  the  specific  gravity  of  a  solid  is 
obtained.  Describe  the  experiment  of  weighing  water.  What  is  stated 
of  Archimedes  and  the  crown? — 117.  Give  a  condensed  rule  for  finding 
the  specific  gravity  of  a  body  heavier  than  water.  Illustrate  by  an  ex- 
ample. Give  what  is  known  as  the  method  by  the  flask.  How  is  the  spe- 
cific gravity  of  a  substance  soluble  in  water  determined  ?  How  that  of 
a  body  lighter  than  water  and  insoluble  in  it?  How  that  of  substances 
lighter  than  water  and  soluble  in  it?  How  can  you  find  the  weight  of 
two  metals  .in  an  alloy  ?  At  what  temperature  should  accurate  determi- 
nations be  made? — 118.  Describe  the  first  method  for  determining  the 
specific  gravity  of  a  liquid.  The  second  method.  What  is  a  hydrom- 
eter ?  How  is  it  used?  Name  some  of  the  varieties  of  hydrometers.  For 
what  is  the  lactometer  used? — 119.  What  is  said  of  tables  of  specific  grav- 
ity? Give  a  few  examples  from  the  table  of  solids.  Give  examples  from 
the  table  of  liquids.  What  is  said  of  the  process  for  determining  the  spe- 
cific gravity  of  gases  ? 

1 


. 


196 


NATURAL  PHILOSOPHY. 


CHAPTER  XII. 

HYDRAULICS. 

120.  Hydraulics. — Hydraulics  teaches  about  liquids  in 
motion,  whether  issuing  from  vessels  or  moving  in  chan- 
nels, of  the  employment  of  water  as  a  source  of  work-power, 
and  of  machines  used  for  raising  water  to  a  height.  If  an 
opening  be  made  in  the  bottom  or  side  of  a  tank  filled  with 
water,  the  liquid  will  flow  through  the  orifice  in  obedience 
to  gravitation,  the  particles  of  liquid  near  the  orifice  being 
pushed  out  by  the  pressure  of  those  around  and  above  them. 
Let  us  examine  more  carefully  some  of  the  phenomena 
connected  with  this  flow  of  liquids  through  an  opening. 

Let  A,  Fig.  165,  represent  a  ves- 
sel of  water  having  three  open- 
ings, B,  C,  and  D,  C  being  equi- 
distant from  B  and  D.  Sup- 
pose B  and  D  are  closed  and 
water  flows  from  C;  it  is  plain 
that  the  rapidity  with  which 
it  issues  must  depend  upon 
the  pressure,  and  consequently 
upon  the  height  of  the  liquid 
above  the  opening ;  and  since 
this  level  continually  falls,  the  pressure  of  the  liquid  and 
the  velocity  of  the  flow  diminish  also.  If  the  level  be  not 
maintained  by  replenishing  a  vessel,  it  takes  twice  as  long 
to  empty  it  as  it  otherwise  would  do. 

Again,  suppose  the  orifice  B  is  one  foot  below  the  stir- 


Fig.  166. 


HYDKAULICS.  197 

face  of  the  water,  and  that  the  pressure  there  causes  a  cer- 
tain quantity,  say  a  litre,  to  flow  out  in  one  minute ;  if  we 
want  the  water  to  issue  twice  as  fast,  say  two  litres  a  min- 
ute, we  must  make  the  pressure  four  times  as  great ;  or, 
what  is  the  same  thing,  another  opening,  C,  of  the  same 
size  must  be  made  four  feet  below  the  level  of  the  water. 
For  the  discharge  of  three  litres  a  minute  the  pressure  must 
be  nine  times  as  great ;  for  a  flow  of  four  litres  a  minute 
the  force  must  be  sixteen  times  as  great,  and  so  forth,  in 
the  proportion  of  squares.  The  reason  for  this  is  that  to 
move  double  the  number  of  water  particles  would  require 
double  the  force  if  they  moved  with  only  the  same  veloc- 
ity ;  but  because  twice  as  many  have  to  press  through  the 
same-sized  opening  in  the  same  time,  each  must  move  with 
double  speed,  and  hence  the  force  must  again  be  doubled ; 
but  two  doublings  are  equivalent  to  a  fourfold  increase. 

When  a  liquid  descends  from  an  opening  in  the  side  of  a 
vessel,  it  follows  the  path  of  a  projectile  (§  67).  In  Fig. 
165  the  water  is  represented  as  spouting  farthest  horizon- 
tally from  the  orifice  C,  in  accordance  with  the  law  that  a 
stream  will  spout  to  the  greatest  distance  from  an  opening 
half-way  between  the  surface  and  the  bottom  of  the  liquid. 
If  B  and  D  are  equidistant  from  C,  the  water  issuing  from 
them  will  strike  the  ground  at  the  same  distance  from  the 
foot  of  the  vessel,  A. 

The  amount  of  the  water  dis- 
charged depends  upon  the  size 
of  the  orifice  and  the  velocity 
of  the  stream.  For  any  given 
time  the  rule  for  finding  the 
quantity  discharged  is  as  fol- 
lows: Multiply  the  area  of  the 
'orifice  by  the  velocity  per  sec- 
ond,  and  this  product  by  th'e 


198  NATURAL  PHILOSOPHY. 

number  of  seconds.  The  shape  of  the  aperture  through 
which  the  water  flows  has  also  a  marked  influence  on  the 
volume  discharged.  A  funnel-shaped  tube  having  a  circu- 
lar section,  Fig.  166,  discharges  more  liquid  in  a  given  time 
than  an  opening  of  any  other  shape. 

121.  Water-Clocks. — The  ancients  took  advantage  of  this  regular 
flow  of  water  through  openings  to  measure  time  before  the  invention  of 
clocks  and  watches.     The  water-clocks,  or  clepsydra,  as  they  were  called, 
were  analogous  in  principle  to  the  common  sand-glass.     Ctesibus,  a  cele- 
brated Greek  philosopher  of  Alexandria,  about  250  B.C.,  contrived  a  most 
ingenious  form  of  this  instrument.     Water  flowed  as  tears  from  the  eyes 
of  a  statuette  which  seemed  to  be  deploring  the  passage  of  time ;  the  tears 
gradually  filled  a  reservoir,  and  raised  a  floating  figure  which  pointed  to 
the  hours  marked  on  a  scale.     This  reservoir  emptied  itself  by  means  of  a 
siphon  arranged,  as  in  the  cup  of  Tantalus  (§  143),  once  every  twenty- 
four  hours,  and  the  discharge  of  the  water  worked  mechanism  which  indi- 
cated the  day  and  the  month. 

122.  Flow  of  Liquids  through  Tubes.— The  flow  of  liquids 
through  long  tubes  and  pipes  is  considerably  affected  by 
friction.     An  inch  tube  200  feet  long,  connected  horizontal- 
ly with  a  reservoir,  will  discharge  water  only  one  quarter 
as  fast   as   an  inch  orifice   in  the   side  of  the  reservoir. 
Sudden  turns  in  a  pipe  should  be  avoided,  because  they  oc- 
casion so  much  friction  against  the  sides  of  the  pipe  and 
among  the  particles  of  water  by  disturbing  the  regularity 
of  the  current.     In  the  entrance  of  the  arteries  into  the 
brain,  in  order  to  prevent  the  blood  from  flowing  too  rap- 
idly into  this  organ,  there  are  sudden  turns  in  the  arteries 
to  retard  the  blood ;  and  in  grazing  animals,  since  there  is 
special  danger  that  the  blood  will  flow  too  freely  to  the 
brain  as  the  head  is  held  down  in  eating,  there  is  a  special 
provision  to  prevent  this  in  a  net-work  of  arteries.     If  the 
arteries  of  the  brain  in  such  animals  were  straight  tubes, 
they  would  continually  be  dying  of  congestion  of  the  brain 
or  of  apoplexy. 


HYDRAULICS. 


199 


Friction  of  liquids  in  a  small  pipe  is  greater  in  proportion 
to  its  size  than  in  a  large  pipe.  In  a  pipe  an  inch  in  diam- 
eter water  moves  only  one  fifth  as  fast  as  in  a  tube  two 
inches  in  diameter.  This  may  be  made  clear  by  Fig.  167, 
which  represents  the  area  of  a  small 
tube  inside  of  the  area  of  a  tube  hav- 
ing twice  its  diameter.  Suppose  the 
eifect  of  the  friction  in  the  large  tube 
to  extend  in  to  a.  In  the  small  one 
it  will  extend  in  as  far — that  is,  to  b. 
But  e  a  is  about  five  times  as  long  as 
e  5,  so  that  there  is  fully  five  times  Fig.i6T. 

more  water  uninfluenced  by  friction  in  the  large  tube  than 
in  the  smaller  one. 

Friction  in  Streams. — The  retarding  effect  of  friction  is  very  obvious  in 
brooks  and  rivers.  The  water  in  the  middle  of  a  stream  runs  much  more 
rapidly  than  it  does  near  its  banks.  When  a  river  is  very  shallow  at  its  sides, 
the  water  there  scarcely  moves,  though  in  the  middle  the  water  may  be 
running  at  a  rapid  rate.  A  tide,  therefore,  flowing  up  a  river,  moves  more 
freely  near  its  banks  than  it  does  in  the  middle  of  the  stream,  because  it 
there  meets  with  less  resistance  from  the  downward  current.  Water  moves 
less  rapidly  at  the  bottom  of  a  river  than  at  the  surface.  For  this  reason, 
if  a  stick  be  so  loaded  at  one  end  as  to  stand  upright  in  water,  in  the  cur- 
rent of  a  river  its  upper  end  will  be  carried 
along  faster  than  its  lower  end,  and  therefore 
it  will  incline  forward,  as  in  Fig.  1 68.  As  the 
sea  rolls  in  over  a  beach,  each  wave  at  length 
pours  over  its  crest  and  breaks,  because  the 
lower  part  of  the  wave  is  retarded  by  friction 
on  the  beach.  Were  it  not  for  the  constant 
retardation  of  friction  at  the  sides  and  bottom 
of  rivers,  and  at  their  bends,  those  rivers  which 

have  their  rise  at  a  considerable  height  above  the  level  of  the  sea  would  ac- 
quire an  immense  velocity.  Thus  the  Rhone,  drawing  its  waters  from  1000 
feet  above  the  level  of  the  ocean,  would  pour  them  forth  with  the  velocity 
of  water  which  had  fallen  perpendicularly  the  same  height — that  is,  at  the 
rate  of  170  miles  an  hour — did  not  friction  continually  diminish  the  velucit  v. 


\ 


200  NATURAL   PHILOSOPHY. 

123.  "Waves. — Waves  are  generally  formed  by  the  fric- 
tion of  air  upon  water.  As  soon  as  any  portion  of  water  is 
raised  above  the  general  surface,  it  tends  by  gravity  to  fall 
to  a  level  with  the  water  around  it,  and  in  so  doing  the 
portion  next  to  it  is  forced  upward,  forming  another  wave ; 
thus  one  wave  produces  another,  each  one  being  smaller 
than  the  preceding,  till  at  length  the  motion  is  wholly  lost. 
This  is  always  the  process  when  the  cause  of  the  motion  is 
a  single  impulse,  as  when  a  stone  is  dropped  into  the  wa- 
ter. But  when  the  waves  are  produced  by  a  succession  of 
impulses,  as  by  the  wind,  they  are  mostly  of  the  same  size. 
It  is  quite  a  common  notion  that  the  water  moves  forward 
as  rapidly  as  the  waves  appear  to  do;  but  the  water  really 
remains  nearly  stationary,  rising  and  falling,  while  merely 
the  form  of  the  wave  advances.  The  same  wave  is  made 
up  continually  of  a  succession  of  different  portions  of  water, 
or  rather  it  is  a  succession  of  different  waves.  This  is  very 
well  illustrated  by  the  waving  of  a  rope  or  carpet.  In  an 
open  sea  a  wave  slopes  regularly  on  either  side ;  but  when 
it  comes  near  the  shore,  for  the  reason  given  in  §  122,  it 
grows  more  and  more  nearly  perpendicular  on  the  side  tow- 
ard the  shore,  till  at  length  it  falls  over;  and  if  it  be  very 
large,  the  roar  thus  caused  by  its  breaking  is  heard  to  a 
great  distance. 

Height  of  Waves. — "So  awful,"  says  Dr.  Avnott,  "is  the  spectacle  of 
a  storm  at  sea  that  it  is  generally  viewed  through  a  medium  which  biases 
the  judgment;  and,  lofty  as  waves  really  are,  imagination  pictures  them 
loftier  still.  Few  waves  rise  more  than  fifteen  feet  above  the  ordinary  sea- 
level,  which,  with  the  fifteen  feet  that  its  surface  afterwards  descends  below 
this,  gives  thirty  feet  for  the  whole  height  from  the  bottom  of  any  water- 
valley  to  an  adjoining  summit.  This  proposition  is  easily  verified  by  ob- 
serving at  what  height  on  a  ship's  mast  the  horizon  remains  always  in  sight 
over  the  top  of  the  near  waves  at  the  time  when  she  reaches  the  bottom 
of  the  hollow  between  two  waves.  Allowance  must  of  course  be  made  for 
accidental  inclinations  of  the  vessel,  and  for  her  sinking  in  the  water  to 


HYDRAULICS.  201 

much  below  her  water-line.  The  spray  of  the  sea,  driven  along  by  the 
violence  of  the  wind,  is  of  course  much  higher  than  the  summit  of  the 
liquid  wave ;  and  a  wave  coming  against  an  obstacle  may  dash  to  an  eleva- 
tion much  greater  still.  At  the  Eddystone  Light-house,  reared  on  a  soli- 
tary rock  ten  miles  from  the  land,  a  wave  which  has  been  growing  from  far 
across  the  Atlantic  often  dashes  above  the  lantern  at  the  summit,  which  is 
about  ninety  feet  high." 

124.  The  Tides. — The  rise  and  fall  of  the  water  of  the 
ocean,  called  tide,  result  from  the  attraction  of  the  moon. 
The  inoon  actually  lifts  the  water  towards  itself.  The 
attraction  of  the  sun  sometimes  increases  and  sometimes 
diminishes  the  tides,  according  to  its  position  in  relation 
to  the  moon  and  the  earth.  If  the  land  were  as  movable 
as  the  water,  or,  in  other  words,  if  its  particles  were  held 
together  by  no  stronger  attraction  than  those  of  water, 
there  would  be  the  same  motion  over  the  surface  of  the 
earth,  when  in  its  revolution  successive  portions  of  it  pre- 
sent themselves  towards  the  moon. 

When  the  flood-tide  returning  from  the  sea  meets  the  out- 
ward current  of  a  river  flowing  into  a  gradually  narrow- 
ing arm  of  the  sea,  the  immensely  powerful  mass  of  the 
ocean  moves  inland,  like  an  almost  vertical  wall,  with  irre- 
sistible force.  Such  a  heaping-up  of  the  waters  where  the 
two  currents  meet  is  called  the  bore.  This  phenomenon  is 
seen  to  a  remarkable  degree  in  the  branches  of  the  Ganges; 
its  roaring  is  heard  long  before  its  arrival,  and  all  small 
vessels  seek  positions  of  safety  on  shore,  while  even  large 
ships  are  occasionally  damaged  by  its  resistless  sweep. 
At  Calcutta  the  wrater  sometimes  rises  five  feet  instanta- 
neously, and  the  huge  wave  rolls  on  at  the  rate  of  fifteen 
miles  an  hour.  The  effects  of  a  strong  tide  are  also  seen 
in  certain  places  where  the  configuration  of  the  coast  com- 
pels the  incoming  water  to  rise  to  great  heights.  In  the 
Bay  of  Fundy  the  returning  tide  advances  with  such  ra- 


202  NATURAL   PHILOSOPHY. 

pidity  that  a  person  on  horseback  who  incautiously  ven- 
tures too  near  can  scarce  escape  being  overwhelmed. 

125.  Relation  of  Bulk  to  the  Resistance  of  Liquids  and 
Gases. — You  have  already  seen,  in  §  64,  that  the  greater 
the  surface  of  a  body  in  proportion  to  its  weight,  the 
greater  the  resistance  of  the  air  to  its  motion.  This  truth, 
which  applies  to  liquids  as  well  as  to  gaseous  substances, 
explains  the.  fact  that  small  bodies  meet  with  proportion- 
ately more  resistance  than  large  ones. 
The  body  B,  Fig.  169,  is  made  up  of  eight 
cubes  of  the  size  of  the  cube  a,  that  is,  it 
has  eight  times  the  quantity  of  matter. 
Now,  if  B  were  moving  through  air  or 
Fig.  169.  water,  any  one  of  its  sides  pushing  the 

water  before  it  would  meet  with  only  four  times  as  much 
resistance  as  a  would,  for  its  surface  is  only  four  times  as 
large,  although  the  body  is  eight  times  as  large  as  a.  And 
the  greater  the  difference  of  size,  the  greater  is  the  differ- 
ence of  resistance.  If  B  were  a  cube  twenty-seven  times 
as  large  as  a,  it  would  meet  with  only  nine  times  as  much 
resistance.  This  explains  why  shells  and  cannon-balls  can 
be  thrown  much  farther  than  bullets  and  small  shot.  The 
sportsman  does  not  throw  away  his  shot  by  foolishly  aim- 
ing at  birds  at  great  distances,  and  yet  shells  and  large 
cannon-balls  can  be  thrown  a  distance  of  several  miles. 
The  difference  is  not  in  the  degree  of  velocity  which  the 
powder  produces,  but  in  the  resistance  of  the  air.  For 
the  same  reason  rain  falls  with  greater  rapidity  than  driz- 
zling mist. 

Since  liquids  and  aeriform  substances  resist  solids  in  motion  in  propor- 
tion to  the  amount  of  surface  which  the  solids  present  to  them,  when 
they  strike  against  solids  they  cause  motion  in  them  in  proportion  to  the 
amount  of  surface  acted  upon.  Thus  a  violent  wind  which  could  not 
move  a  lump  of  tin  could,  nevertheless,  raise  a  sheet  of  it,  or  tear  up  a 


HYDRAULICS.  203 

roofing  of  it  if  permitted  to  get  beneath.  Clouds  of  sand  are  raised  into 
the  air  in  the  deserts  of  Africa,  although  the  particles  are  of  the  same  ma- 
terial as  stones,  and  therefore  have  the  same  specific  gravity.  For  the 
same  reason  dust,  feathers,  the  down  and  pollen  of  flowers,  etc.,  are  blown 
about,  although  they  are  heavier  than  the  air.  A  pebble  is  moved  more 
easily  by  a  current  of  water  than  a  stone,  because  it  has  a  larger  surface, 
in  proportion  to  its  weight,  to  be  acted  upon  by  the  water.  For  the  same 
reason  sand  is  moved  more  easily  than  pebbles,  and  fine  mud  than  sand, 
though  stones,  pebbles,  sand,  and  mud  may  all  be  of  the  same  material. 
This  explains  why  you  find  mud  where  the  current  is  slow,  sand  where  it 
is  faster,  pebbles  and  stones  where  it  is  still  faster,  and  where  the  current 
is  exceedingly  rapid  you  will  find  nothing  but  large  rocks — sand,  pebbles, 
and  stones  not  being  able  to  resist  its  force.  For  the  same  reason,  in 
the  process  of  winnowing,  the  chaff  is  carried  away  by  the  wind;  while  the 
grain,  presenting  less  surface  in  proportion  to  its  weight  to  be  acted  upon 
by  the  air,  falls  to  the  floor. 

Influence  of  Shape  on  Resistance  of  Liquids  to  Solids. — 
The  resistance  of  air  or  water  to  a  flat  surface  is  greater 
than  to  a  convex  one,  because  the  latter  readily  turns  the 
particles  aside.  Thus,  a  concave  surface  is  resisted  much 
more  than  a  flat  one,  because  the  particles  of  the  air  or 
water  cannot  so  easily  escape  sideways.  Fishes  are  of 
a  spindle-like  and  slender  shape,  that  they  may  offer  as 
little  resistance  as  possible  to  the  water.  It  is  for  this 
reason  that  a  fish  has  no  neck,  otherwise  the  upper  por- 
tion of  its  body  would,  from  the  resistance  of  the  water 
striking  against  it,  prove  a  serious  impediment  to  rapid- 
ity of  motion.  Mankind  has  in  some. measure  imitated 
the  shape  of  fishes  in  their  boats  and  ships.  Boats  which 
are  intended  to  bear  light  burdens  and  go  swiftly  are  made 
very  long  and  narrow.  The  webbed  feet  of  water-fowls, 
when  they  are  moved  forward,  are  folded  up  so  as  to 
meet  with  as  little  resistance  as  possible;  but  when  they 
are  moved  backward  they  are  spread  out  so  as  to  press 
against  the  water  a  broad  concave  surface.  For  the  same 
reason  the  wings  of  a  bird  are  made  convex  upward 


204  NATURAL   PHILOSOPHY. 

and  concave  downward ;  and  when  it  moves  its  wing  up- 
ward it  cuts  the  air  somewhat  edgewise,  but  in  moving 
it  downward  it  presses  directly  with  the  whole  concave 
surface. 

I  126.  Machines  for  Raising  Water. — A  great  variety  of 
contrivances  for  raising  water  from  a  lower  to  a  higher 
level  have  been  devised,  some  of  which  are  based  on  a 
simple  application  of  one  or  more  of  the  six  simple  ma- 
chines described  in  Chapter  VI.  Such  are  the  well-sweep, 
acting  on  the  principle  of  the  lever,  and  the  rope  and 
bucket  suspended  from  a  wheel  and  axle.  An  old  system 
of  raising  water  is  by  means  of  a  succession  of  buckets 
attached  to  an  endless  rope  passing  over  two  wheels,  so 
that  the  buckets  fill  as  they  are  carried  over  the  lower 
wheel  and  discharge  as  they  pass  over  the  top  wheel. 
The  chain  pump,  used  in  many  parts  of  this  country,  is 
somewhat  similar ;  but  the  buckets  are  replaced  by  flat 
disks  of  metal,  which  are  drawn  up  through  a  long  tube 
or  barrel,  like  loose-fitting  pistons,  and  raise  an  abundant 
stream  of  water.  The  celebrated  philosopher  Archimedes 
invented  a  simple  machine,  known  as  Archimedes's  screw, 
by  means  of  which  water  may  be  readily  raised  to  a  mod- 
erate elevation.  It  consists  of  a  tube  open  at  both  ends, 
wound  spirally  around  an  inclined  cylinder  as  represented 

in  Fig.  170.  The  low- 
er end  of  the  tube  dips 
below  the  water ;  on 
revolving  the  cylinder 
the  open  end  scoops  up 

Fig.  1TO. 

water,  and  when  it  has 
turned  half-way  around,  the  point  D  is  lower  than  the  end 

C,  and,  in  obedience  to  gravitation,  the  water  descends  to 

D.  On  continuing  to  revolve  the  screw,  the  water  rises  to 
the  top,  B,  as  if  drawn  up  an  inclined  plane.    Archimedean 


HYDEAULICS.  205 

screws  are  still  used  in  Holland  for  draining,  and  are  gen- 
e rally  driven  by  windmills. 

The  various  kinds  of  pumps  used  for  raising  water,  being 
dependent  upon  the  principles  of  pneumatics,  will  be  de- 
scribed in  the  chapter  treating  of  that  topic. 

12V.  Water-wheels.  —  Water  flowing  in  streams  having 
considerable  descent  affords  motive  power  of  first  impor- 
tance. It  can  be  made  to  perform  work  through  the 
agency  of  water-wheels.  These  wheels  are  of  three  princi- 
pal kinds — the  Undershot  wheel,  the  Overshot  wheel,  and 
the  Turbine.  The  undershot  wheel  consists  of  a  wheel  re- 
volving on  an  axle,  and  having  a  number  of  float-boards 
attached  to  its  circumference,  Fig.  171.  These  float-boards 


Fig.  171. 

dip  into  the  water,  which,  by  its  momentum,  drives  the 
wheel  around,  the  velocity  depending  upon  the  height  of 
the  fall  of  water.  In  overshot  wheels  the  float-boards  are 
shut  in  by  flat  sides,  so  as  to  form  buckets  round  the  wheel 
into  which  the  water  is  allowed  to  fall  at  the  top  of  the 
wheel,  Fig.  172.  In  this  wheel  the  water  acts  almost  solely 
by  its  weight ;  as  the  wheel  revolves,  the  buckets,  filled  at 
the  top,  descend,  and  discharge  the  water,  so  that  by  the 

12 


206 


NATUEAL  PHILOSOPHY. 


Fig.  1T2. 


time  they  begin  to 
rise  on  the  opposite 
side  they  are  empty. 
When  the  water  is 
received  half-way  up 
the  wheel,  or  higher, 
the  arrangement  is 
called  a  Breast 
Wheel. 

The  Turbine  pre- 
sents a  very  differ- 
ent appearance  :  it 
consists  of  a  hori- 
zontal wheel  divided 
into  compartments 
by  curved  lines,  as 
shown  in  that  portion  of  the  cut  (Fig.  173)  without  the 
heavy  circle.  Within  this  is 
fitted  a  fixed  cylinder,  also 
divided  into  compartments 
similar  to  those  in  the  wheel, 
but  running  in  the  opposite 
direction.  Water,  from  a 
height,  enters  a  tube  con- 
nected with  this  cylinder, 
and,  following  the  course 
given  by  the  curved  lines, 
strikes  against  the  partitions 
of  the  wheel,  causing  it  to 
revolve  about  a  vertical  axis.  Owing  to  the  pressure  of 
the  water  within  the  tube,  and  to  its  striking  the  parti- 
tions nearly  at  right  angles,  turbines  turn  to  account  a 
larger  proportion  of  the  motive  power  (four  fifths)  than 
any  other  wheel. 


Fig.  173. 


HYDRAULICS. 


207 


Somewhat  similar  in 
principle  is  the  so-called 
Barker's  Mill ;  it  consists 
of  a  vertical  cylinder  ar- 
ranged in  a  frame  in  such 
a  way  that  it  can  revolve 
upon  the  point  upon  which 
it  rests.  Water  running  i  I 
into  the  cylinder  escapes 
by  two  arms  having  holes 
on  the  alternate  sides;  by 
this  arrangement  the  re- 
action upon  the  issuing 
water  makes  the  cylinder 

revolve  rapidly,  causing  the  ends  of  the  arms  to  revolve 
as  represented  in  the  figure  (Fig.  174). 


QUESTIONS. 

120.  Of  what  does  Hydraulics  teach?  Describe  some  of  the  phenom- 
ena connected  with  the  flow  of  liquids  through  an  opening.  What  is  the 
path  of  a  liquid  issuing  from  a  lateral  opening  ?  Upon  what  depends  the 
amount  of  water  discharged? — 121.  What  is  said  of  water-clocks  ? — 122. 
How  does  friction  affect  the  flow  of  liquids  through  long  tuhes  ?  What 
is  said  of  the  effect  of  friction  in  brooks  and  rivers?  In  what  part  of  a 
stream  does  the  water  move  most  rapidly  ?  Explain  the  formation  and 
breaking  of  the  crest  of  waves  rolling  over  a  beach.  What  is  said  of  the 
velocity  of  rivers'as  affected  by  friction  ? — 123.  Explain  the  formation  of 
waves.  What  is  it  that  really  advances  in  the  forward  movement  of  a 
wave?  Give  the  comparison  mentioned.  What  is  said  of  the  height  of 
waves? — 124.  What  causes  tides?  What  is  a  bore?  Mention  some  places 
where  its  effects  are  noteworthy. — 125.  Illustrate  the  relation  of  bulk  to 
the  motion  of  solids  produced  by  moving  gases  and  liquids.  What  is  said 
of  the  opposition  of  gravitation  to  water  and  air  in  moving  solids  ?  What 
difference  does  the  presence  of  obstacles  make  in  the  relation  of  force  to 
velocity  ?  What  is  said  of  the  relation  of  shape  to  velocity  ?  What  is  said 
of  the  shape  of  fishes  ?  What  is  said  of  the  shape  of  boats  ?  What  of 


208  NATURAL   PHILOSOPHY. 

the  management  of  the  webbed  feet  of  water-fowls  ?  What  of  the  wings 
of  birds? — 126.  What  is  said  of  machines  for  raising  water?  Describe 
Archimedes's  screw.  — 127.  Name  the  principal  kinds  of  water-wheels. 
Describe  the  undershot  wheel,  and  explain  its  action.  Also  the  other 
forms.  The  turbine.  Describe  Barker's  Mill. 


CHAPTER  XIII. 

PNEUMATICS. 

128.  What  Pneumatics  Teaches.  —  Hydrostatics,  as  yon 
have  learned  in  the  preceding  chapters,  treats  of  the  press- 
ure and  equilibrium  of  liquids,  and  Hydraulics  of  the  laws 
governing  their  motion.  Pneumatics  is  that  branch  of  phys- 
ics which  treats  of  the  same  phenomena  in  air  and  other 
aeriform  bodies.  The  name  is  derived  from  a  Greek  word 
signifying  breath  or  air,  just  as  the  term  hydrostatics  comes 
from  the  Greek  for  icater.  In  explaining  the  laws  of"  liquid 
level,"  "  equal  pressure  in  all  directions,"  and  of  "  pressure 
varying  with  the  depth,"  we  have  studied  the  phenomena 
with  reference  only  to  water  as  the  most  convenient  liquid ; 
but  these  laws  hold  good  with  all  other  liquids.  In  like 
manner,  the  laws  which  we  are  about  to  teach  concerning 
common  air  are  equally  applicable  in  the  case  of  all  other 
gases  under  similar  circumstances. 

Air  Material  and  has  Weight.  — That  air  is  a  material 
substance  has  been  shown  in  §  12,  where  its  impenetrabil- 
ity was  demonstrated.  It  is  much  less  dense  than  water 
by  reason  of  a  greater  separation  and  repulsion  of  its  par- 
ticles; but  analogous  phenomena  are  observed  with  both 
these  fluids. 

For  example,  if  you  fill  an  India-rubber  bag  with  water 
and  tie  its  mouth,  you  cannot  flatten  it  by  pressure;  and 


PNEUMATICS.  209 

if  you  blow  into  it  until  it  is  distended  and  again  fasten 
its  moutl},  it  remains  bulky,  forming  what  is  known  as  an 
air-cushion.  Life-preservers  and  foot-balls  are  examples  of 
such  air-cushions. 

Then,  again,  the  resistance  offered  by  air  to  motion,  as  in 
fanning,  the  power  possessed  by  currents  of  air  to  move 
light  as  well  as  heavy  objects,  and  the  flight  of  birds  in 
the  air,  all  prove  the  material  nature  of  air.  That  air 
has  weight  can  be  proved  by 
weighing  it  as  you  would  any 
other  substance.  Let  a  hol- 
low globe,  A,  Fig.  175,  having 
a  neck  with  a  stop-cock,  B,  be 
emptied  of  air  and  weighed. 
When  you  open  the  stop-cock, 
and  let  in  the  air,  the  other 
beam  of  the  scale  will  rise,  be- 

Jb  lg.  175. 

cause  the  globe  is  heavier  than 

it  was  before.  The  additional  weight  required  to  make  the 
scales  balance  will  indicate  the  weight  of  the  air  which  the 
globe  contains.  It  is  about  one  eight-hundredth  (y^)  of 
the  weight  of  the  same  volume  of  water.  How  the  globe 
can  be  emptied  of  the  air  will  be  shown  in  another  part  of 
this  chapter  (see  §  134). 

129.  Air  Attracted  by  the  Earth. — The  weight  of  the  air 
is  simply  the  result  of  the  attraction  of  the  earth  (§  27). 
Air  is  attracted  by  the  earth  in  the  same  manner  as  water ; 
and  the  water  takes  its  place  below  air  because  it  is  attract- 
ed more  strongly  than  the  air.  If  you  put  into  a  bottle 
mercury,  water,  and  oil,  the  mercury  will  lie  at  the  bottom, 
because  it  is  more  strongly  attracted  by  the  earth  than 
the  other  fluids.  The  water  will  be  next,  then  the  oil, 
and  lastly,  over  all,  the  air,  that  being  less  attracted  than 
any  of  the  other  substances.  This  attraction  of  the  air  by 


210 


NATURAL  PHILOSOPHY. 


the  earth  is  the  origin  of  the  chief  phenomena  of  Pneu- 
matics. 

Why  Some  Things  Fall  and  Others  Rise  in  Air. — Most  substances  fall 
in  air  for  the  same  reason  that  very  heavy  substances  sink  in  water.  They 
fall  because  the  earth  attracts  them  more  strongly  than  it  does  the  air. 
The  reason  that  some  substances  rise  in  air  is  precisely  the  same  as  that 
given  in  §  113  for  the  rising  of  substances  in  water.  The  air,  being  attract- 
ed more  strongly,  pushes  them  up  to  get  below  them,  as  cork  or  wood  is 
pushed  up  by  water.  Thus  a  balloon  filled  with  hydrogen  gas  rises  in  air 
for  the  same  reason  that  a  bladder  filled  with  air  rises  in  water. 

130.  Thickness  of  the  Earth's  Air  -  Covering. — -The  air 
makes  a  covering  for  the  earth  about  fifty  miles  deep.  If 
the  earth  were  represented  by  a  globe  a  foot  in  diameter, 
the  air  might  be  represented  by  a  covering  a  tenth  of  an 
inch  in  thickness.  The  line  #,  Fig.  176,  shows  the  curve  of 


Fig.  1T6. 

the  surface  of  such  a  globe,  and  the  space  between  a  and 
b  represents  the  comparative  thickness  of  the  covering  of 
air.  This  is  ascertained  by  calculation  from  the  press- 
ure of  the  air  upon  the  earth  in  the  same  manner  as  the 
depth  of  water  is  calculated  from  the  pressure  which  i.ti 
exerts.  • 

The  earth  flies  on  in  its  yearly  journey  around  the  sun  $t  the  rate  of  11  CO 
miles  per  minute,  and  yet  it  holds  on  to  this  loose  airy  robe  by  its  attrac- 
tive force,  so  that  not  a  particle  of  it  escapes  into  the,  surrounding  ether. 
Of  itself  it  is  disposed  to  escape ;  and  it  would  do  so,  and  be  diffused 
through  space,  if  the  attraction  of  the  earth  for  it  wejre  suspended. 

131.  Compressibility  of  Air. — In  considering  the  influence 
of  gravitation  upon  air,  it  must  be  remembered  that  air  is 
very  compressible,  while  water  is  very  nearly  incompressi- 
ble (§  1 7).  While,  therefore,  in  a  body  of  water  the  particles 


PNEUMATICS.  211 

arc  very  little  nearer  together  at  the  bottom  than  at  the 
surface,  the  particles  of  the  air  are  much  nearer  together  at 
the  surface  of  the  earth  than  at  a  distance  from  it.  All 
the  particles  of  the  air  being  attracted  or  drawn  towards 
the  earth,  those  below  are  pressed  together  by  the  weight  of 
those  above.  The  air  therefore  becomes  more  rarefied  as  we 
leave  the  surface  of  the  earth,  and  in  the  outer  regions  of  the 
sea  of  air  it  is  too  rare  to  support  life.  Even  at  the  tops  of 
very  high  mountains,  or  the  heights  sometimes  reached  by 
balloons,  disagreeable  effects  are  often  experienced  from  the 
rarity  of  the  air.  The  air  has  been  compared,  in  regard  to 
its  varying  density  at  different  heights,  to  a  heap  of  some 
loose  compressible  substance ;  as,  for  example,  cotton-wool, 
which  is  quite  light  at  the  top,  but  is  pressed  more  and 
more  compactly  as  you  go  towards  the  bottom.  Hydro- 
gen gas  is  only  one  fifteenth  as  heavy  as  air  at  the  sur- 
face of  the  earth ;  and  therefore  the  hydrogen  balloon  rises 
till  it  reaches  a  height  where  the  air  is  so  rare  that  the 
balloon  is  of  the  same  weight  with  an  equal  bulk  of  air, 
and  there  it  stops. 

132.  Similarity  of  Aeriform  Substances  and  Liquids. — 
You  have  learned  in  §  17  in  what  the  air  and  gases  differ 
from  liquids.  But  in  one  very  important  respect  they  are 
alike — viz.,  the  mobility  of  their  particles.  Hence  pressure 
in  air,  as  well  as  in  water,  is  equal  in  all  directions,  so 
that  in  the  experiment  with  the  bladdeiyin  §  107,  it  makes 
no  difference  in  the  result  whether  it  be  filled  with  water 
or  air.  For  the  same  reason,  pressure  is  in  proportion  to 
the  depth  in  aeriform  substances  as  well  as  in  liquids,  and 
the  laws  of  specific  gravity  apply  to  the  one  as  well  as  to 
the  other. 

You  are  now  prepared  to  understand  the  results  of  the 
action  of  gravitation  upon  air  and  the  gases  •  or,  in  other 
words,  the  principal  phenomenon  of  Pneumatics. 


212  NATUKAL   PHILOSOPHY. 

133.  Pressure  of  the  Atmosphere. — The  amount  of  the 
pressure  of  the  atmosphere  is  very  readily  estimated,  by  a 
process  which  we  will  explain  in  another  part  of  this  chap- 
ter. It  has  been  ascertained  that  the  atmosphere  presses 
with  a  weight  of  fifteen  pounds  on  every  square  inch. 
When  you  extend  your  outspread  hand  horizontally  in  the 
air,  you  feel  no  pressure  upon  it,  notwithstanding  it  sustains 
a  pressure  of  some  two  or  three  hundred  pounds.  If  your 
hand  be  five  inches  long  and  three  broad,  it  presents  a  sur- 
face of  fifteen  square  inches,  on  every  one  of  which  the  at- 
mosphere is  pressing  with  the  weight  of  fifteen  pounds ; 
that  is,  there  is  a  pressure  on  the  upper  surface  of  your  hand 
of  a  column  of  air  weighing  225  pounds,  and  on  the  lid  of  a 
box  only  thirty  inches  square  there  is  a  pressure  of  13,500 
pounds.  The  whole  pressure  on  the  body  of  a  man  of  com- 
mon size  is  about  fifteen  tons.  But  why  is  it  that  the  lid 
of  the  box  is  not  broken  in,  your  hand  not  borne  down,  and 
your  body  not  crushed  ?  It  is  simply  from  the  fact,  shown 
in  the  previous  chapter  in  regard  to  liquids,  and  in  this 
one  as  to  aeriform  substances,  that  the  pressure  is  equal  in 
all  directions.  The  lid  and  the  outspread  hand  are  there- 
fore balanced  by'  an  upward  pressure  equal  to  the  down- 
ward, and  the  body  sustains  an  equal  pressure  on  all  sides. 
If  the  air  could  be  removed  from  within  the  box,  the  lid 
would  be  crushed  in  ;  if  from  under  the  hand,  that  would 
be  borne  down  ;  and  if  from  one  side  of  the  body,  the  body 
would  be  forced  violently  in  that  direction  till  it  met  with 
an  opposing  pressure. 

But  besides  this  equal  pressure  of  the  air  on  all  sides,  air 
exists  within  the  pores  and  interstices  of  all  bodies  that 
are  not  very  dense,  and  its  particles  are  subject  to  the  same 
laws  as  are  those  on  the  outside. 

All  this  can  be  made  clear  to  you  by  experiments  with 
the  air-pump. 


PNEUMATICS. 


213 


Fi 


134.  Air-pump. — Fig.  177  represents  an  air-pump  as  com- 
monly arranged.  A,  B,  are  two  pump -barrels,  the  pistons 
in  which  are  worked  by  means  of  the  handles,  G  and  M. 
These  pumps  are  very  nicely  made,  and  the  frame-work  to 
which  they  are  attached  is  very  strong  and  firm,  so  that  the 
pumps  may  work  evenly.  J  is  a  bell-shaped  glass  vessel, 
called  a  receiver,  closed  at  the  top,  but  open  at  the  bottom, 
the  edge  of  which  is  ground  very  true,  so  that  it  may  fit 
exactly  on  the  large,  smooth  metallic  plate.  In  the  mid- 
dle of  the  plate  is  an  opening  which  leads  to  the  pump- 
barrels,  and  it  is  through  this  that  the  air  is  pumped  out 
of  the  glass  receiver,  J.  If  we  wish  to  let  the  air  in  after 


214 


NATURAL  PHILOSOPHY. 


Fig.  178. 


we  have  pumped 
it  out,  we  loosen 
the  screw  at  K,  for 
there  is  a  passage 
from  this  opening 
to  that  in  the  mid- 
dle of  the  plate. 

The  operation  of  the 
air-pump  can  be  made 
clear  by  reference  to 
Fig.  178.  But  one  pump- 
barrel,  a,  is  represented, 
with  a  piston,  c,  work- 
ing in  it.  In  the  pis- 
ton there  is  a  valve, 
i,  opening  upward,  and  also  one  at  b,  at  the  end  of  the  tube  leading  to 
the  centre  of  the  plate  on  which  is  the  receiver,  d.  The  working  of  the 
instrument  is  as  follows :  If  the  piston,  c,  be  forced  down,  the  air  under 
it,  being  compressed,  will  close  the  valve  at  6,  and  will  rush  upward  through 
the  valve  i  in  the  piston.  Let  the  piston  now  be  raised ;  the  resistance  of 
the  air  above  it  will  close  the  valve  t,  while  the  valve  b  will  be  opened  by 
the  air  rushing  from  the  receiver,  d,  through  the  passage,  e,  to  fill  the  space 
between  the  piston  and  b.  You  see,  then,  that  every  time  the  piston  is 
drawn  up  air  passes  out  of  the  receiver  through  the  valve  b  into  the  space 
between  this  valve  and  the  piston.  None  of  this  air  which  has  passed  out 
can  return ;  for  the  moment  you  press  upon  it  by  forcing  downward  the  pis- 
ton, the  valve  6  closes  and  the  air  escapes  through  the  valve  i.  Each  time, 
therefore,  that  you  work  the  piston  up  and  down,  you  pump  some  of  the 
air  out  of  the  receiver ;  and  after  some  time  exceedingly  little  air  will  be 
left  in  it,  and  that,  of  course,  will  be  diffused  throughout  the  receiver.  It 
will  be  rarefied  like  that  in  the  upper  regions  of  the  atmosphere.  With  the 
double-barrelled  air-pump,  shown  in  Fig.  177,  the  operation  is  similar  but 
more  rapid,  because  when  one  piston  is  raised  the  other  is  lowered,  and  the 
action  is  continuous.  L  is  a  gauge  to  indicate  the  completeness  of  the  ex- 
haustion, which  acts  on  the  principle  of  the  barometer. 

135.  Experiments. — When  the  receiver  J  (Fig.  177)  is  full 
of  air,  it  can  be  moved  about  on  the  plate  easily,  and  can  be 


PNEUMATICS.  215 

lifted  from  it.  But  if  you  work  the  pumps  a  few  strokes, 
the  receiver  will  be  firmly  fastened  to  the  plate,  since 
the  air  within,  being  rarefied,  presses  with  little  force  com- 
pared with  the  air  outside.  If  the  pumps  be  worked  for 
some  time,  it  will  be  very  difficult  to  release  the  receiver 
from  the  pressure  without  breaking  it.  But  turn  the  screw, 
K,  admitting  the  air,  and  the  equality  of  the  pressure  within 
and  without  is  at  once  restored.  Remove  this  large  re- 
ceiver, and  place  a  small  glass  jar,  open  at  both  ends,  on 
the  plate,  with  the  hand  covering  the  upper  opening,  as 
represented  in  Fig.  179.  On  exhausting  the  air,  the  hand 
is  so  firmly  pressed  into  the 
glass  that  it  requires  consid- 
erable force  to  disengage  it 
from  the  pressure.  If  we  tie 
a  piece  of  bladder  or  India- 
rubber  over  this  jar,  as  in  Fig. 
180,  and  then  pump  out  the 
air,  the  bladder  is  at  first  pressed  in ;  and  if  we  continue 
to  pump,  it  at  length  bursts  inward  with  a 
loud  report.  It  would  make  no  difference  in 
the  result  of  the  experiment  if  the  jar  were 
shaped  as  in  Fig.  181,  for  the  pressure  is  the 
same  in  all  directions.  The  resemblance  be- 
tween air  and  liquids  in  this  respect  may  be 
.  isi.  illustrated  thus :  Suppose  that  a  flat  fish  rests 
against  the  tube  of  a  pump  so  as  to  cover  the  end  with  one  of 
his  sides.  He  feels  no  uncomfortable  pressure,  because  the 
water  in  the  pump  and  that  below  it  press  equally  upon  him. 
If,  however,  the  pressure  of  the  water  in  the  pump  be  sud- 
denly removed  by  the  piston,  the  fish  would  be  pressed  up- 
ward into  the  tube,  just  as  the  bladder  is  pressed  upward 
in  Fig.  181,  or  downward  in  Fig.  180.  The  so-called  "Mag- 
deburg Hemispheres,"  Fig.  182,  illustrate  very  strikingly 


216 


NATURAL   PHILOSOPHY. 


the  pressure  of  the  atmosphere. 
They  consist  of  two  hollow  half- 
globes  of  metal  whose  edges  fit 
very  accurately  upon  each  other. 
The  air  being  exhausted  through 
the  stem  and  a  handle  screwed 
on,  great  force  must  be  exerted  to 
pull  the  hemispheres  apart.  The 
force  required  depends  upon  the 
extent  of  their  surface.  In  the 
famous  experiment  at  Magdeburg, 
in  1654,  by  Otto  von  Guericke, 
Flg-182'  the  inventor  of  the  air-pump,  two 

strong  hemispheres  of  brass  three  feet  in  diameter  were 
employed ;  and  when  he  exhausted  them  on  the  occasion 
of  a  public  exhibition,  it  is  said  that  twenty  coach-horses 
of  the  emperor  were  unable  to  pull  them  asunder ! 

In  the  so-called  "Mercury  Shower,"  we  have  another 
example  of  the  immense  pressure  of  the  atmosphere.  Fig. 
183  represents  a  receiver  with  an  opening  at  the  top. 
mented  in  this  opening  is  a  wooden  cup,  a,  ter- 
minating in  a  cylindrical  piece,  b.  If  mercury 
be  poured  into  the  cup  and  the  air  within  the 
receiver  be  exhausted,  the  mercury  will  be  forced 
through  the  pores  of  the  wood  by  the  external 
air,  and  will  fall  in  a  silver  shower.  A  tall  jar,  c, 
is  placed  there  to  receive  it,  to  prevent  any  of  it 

from  entering  the  opening  in  the  metallic  plate. 


The  boy's  sucker  illustrates  the  pressure  of  the  air.  It  is 
simply  a  circular  piece  of  leather  with  a  string  fastened  to  its 
centre,  as  shown  in  Fig.  184.  When  the  leather  is  moistened 
and  pressed  upon  a  smooth  stone,  it  adheres  by  its  edges  to  the 
stone,  just  as  the  receiver  adheres  to  the  plate  of  the  air-pump 
when  the  air  is  pumped  out.  Many  animals  have  contrivances 


Fig.  184. 


PNEUMATICS. 


217 


of  a  similar  character  to  enable  them  to  walk  in  all  positions,  to  seize  their 
prey,  etc.  The  gecko  and  the  cuttle-fish  furnish  interesting  examples,  as 
noticed  in  Hooker's  Natural  History.  Snails,  limpets,  etc., adhere  to  rocks 
by  a  like  arrangement.  Some  fishes  do  the  same ;  one,  called  the  remora, 
attaches  itself  by  suckers  to  the  side  of  some  large  fish  or  a  ship,  and  thus 
enjoys  a  fine  ride  through  the  water  without  any  exertion  on  his  part. 
In  all  such  cases  it  is  water  instead  of  air  that  makes  the  pressure,  but 
the  principle  is  the  same.  Flies  and  some  other  insects  can  walk  up  a 
smooth  pane  of  glass,  or  along  the  ceiling,  because  their  feet  have  contriv- 
ances similar  in  principle  to  the  boy's  sudker.  The  hind-feet  of  the  wal- 
rus are  constructed  somewhat  like  the  feet  of  the  fly,  enabling  this  huge 
animal  to  climb  smooth  walls  of  ice. 

136.  Density  of  the  Air  Dependent  upon  Pressure. —  The 
fact  that  the  degree  of  the  density  of  the  air  is  dependent 
on  pressure  has  been  already  shown  in  §  131.  The  same 
thing  can  be  shown  in  various  ways  by  experiments  with 
an  air-pump.  If  a  small  bladder  partly  filled  with  air, 
Fig.  185,  and  loaded  with  a  weight  so  as  to  sink  in  water, 
be  placed  in  a  jar  of  water,  and  the  whole  be 
set  under  the  receiver  of  the  air-pump,  on  ex- 
hausting the  air  the  bladder  will  swell  out, 
owing  to  the  expansion  of  the  air,  and  will  rise. 
The  reason  is,  that  the  pressure  being  removed 
from  the  surface  of  the  water,  the  bladder  bears 
only  the  pressure  of  the  water,  and  not  that  of 
the  air  plus  the  water ;  hence  the  air  within 
expands  and  becomes  less  dense.  If  an  In- 
dia-rubber bag  be  partly  filled  with  air,  Fig.  186,  and  put 
under  the  receiver,  when  the  air  is  exhausted  the  bag  is 
relieved  of  pressure,  and  the  air  in  it  becomes  ex- 
panded— that  is,  rarefied.  For  the  same  reason, 
if  a  vessel  with  soap-bubbles  in  it  be  placed  under 
the  receiver,  on  pumping  out  the  air  the  bubbles 
will  become  much  enlarged.  A  very  pretty  ex- 
Fig. i9c.  periment  illustrates  the  same  principle.  Let  an 


Fig.  185. 


218 


NATURAL   PHILOSOPHY. 


egg  with  a  hole  in  its  small  end  be  suspend- 
ed in  a  receiver,  as  represented  in  Fig.  187, 
a  wine-glass  being  placed  beneath  it.  On 
exhausting  the  air,  the  egg  will  run  out  of 
the  shell  into  the  wine-glass,  and  then,  on 
admitting  the  air,  the  larger  part  of  it  will 
run  back  again  into  the  shell.  This  may  be 
explained^as  follows:  The  large  end  of  the 
Fig.  137.  egg  contains  air.  As  soon  as  the  pressure 
of  air  is  removed  from  the  egg,  the  air  in  the  egg  expands, 
forcing  out  the  contents;  but  when  the  air  is  admitted  into 
the  receiver,  the  air  in  the  egg  is  at  once  condensed  to  its 
former  small  bulk  by  the  surrounding  pressure. 

Hydrostatic  Balloon. — The  philosophical  toy  represented 
iu  Fig.  188  illustrates  very  beautifully  the  influence  of 
pressure  upon  the  density  of  the  air.  The  balloon 
in  the  jar  of  water  is  constructed  of  glass,  having 
a  small  orifice  at  its  lower  part.  Water  is  intro- 
duced into  the  balloon,  care  being  taken  to  put  in 
just  enough  to  make  the  balloon  of  a  little  less 
specific  gravity  than  water.  In  that  case  it  will 
rise  to  the  top  of  the  jar,  with  a  very  little  of  its 
top  above  the  surface  of  the  water.  Now  tie  a 
piece  of  India-rubber  cloth  over  the  top  of  the  jar, 
and  the  apparatus  is  complete.  On  pressing  upon 
the  India-rubber  the  balloon  will  descend  in  the 
jar,  and  on  removing  the  pressure  it  will  rise.  The 
explanation  is  as  follows:  The  pressure  upon  the 
India-rubber  is  felt  through  the  whole  body  of  the  water  in 
the  jar,  and  forces  a  little  more  water  into  the  orifice  of  the 
balloon,  condensing  the  air  within  it.  The  balloon  conse- 
quently becomes  heavier,  and,  having  a  greater  specific 
gravity  than  water,  sinks.  But  when  the  pressure  is  re- 
moved, the  condensed  air  in  the  balloon,  by  its  elasticity, 


Fig. 188. 


PNEUMATICS.  219 

returns  to  its  former  bulk,  expelling  the  surplus  water  just 
introduced;  and  the  balloon,  becoming  therefore  as  light  as 
before,  rises. 

137.  Pores  of  Substances  Contain  Air. — We  have  said 
that  the  pores  and  interstices  of  wood,  flesh,  and  a 
great  variety  of  substances  contain  air.    In  all  these 

cases  the  presence  of  the  air  can  be  made  manifest 
by  removing  the  pressure  of  the  surrounding  air, 
and  thus  allowing  the  air  in  these  substances  to  ex- 
pand. If  an  egg  be  placed  in  a  jar  of  water,  Fig. 
189,  under  the  receiver  of  an  air-pump,  on  exhaus- 
tion being  made,  air-bubbles  will  constantly  rise  in  Fig.  is*, 
the  water  from  the  egg.  In  like  manner,  the 
surface  of  a  glass  of  ale,  Fig.  190,  will  be  covered 
with  foam,  the  carbonic-acid  gas  in  it  escaping 
freely  when  the  pressure  of  the  air  upon  it  is  re- 
moved. The  same  thing  may  be  seen  to  some 
extent  even  in  water,  for  it  always  contains  some 
Fig.  190.  air.  For  a  similar  reason  a  shrivelled  apple  will 
become  plump  and  fair  when  the  pressure  of  the  external 
air  is  removed,  but  will  shrink  at  once  to  its  shrivelled  state 
when  the  air  is  admitted  into  the  receiver. 

138.  Elasticity  of  the  Air. — All   the   phenomena   men- 
tioned in  §  136  and  §  137  exhibit  the  elasticity  of  the  air. 
Owing  to  this  property  it  is  always  disposed  to  expand 
when  pressure  is  removed  from  it.     This  is,  most  strikingly 
exhibited  when  the  air  is  much  condensed  by  pressure;  the 
greater  the  condensation,  the  stronger  the  expansive  or  elas- 
tic force.    Fig.  191,  page  220,  represents  an  instrument  called 
the  condenser.     In  the  cylinder,  A  B,  moves  the  piston,  P. 
Air  is  admitted  to  the  cylinder  at  F,  and  into  the  receiver, 
V,  at  G.     The  valve  at  F  prevents  any  air  from  escaping 
from  the  cylinder,  and  the  valve  at  G  prevents  it  from  escap- 
ing from  the  receiver.    The  instrument  operates  thus :  If  the 


220 


NATUKAL   PHILOSOPHY. 


Fig. 191, 


piston  be  pressed  downward,  the  compress- 
ed air  in  the  cylinder  shuts  the  valve  F 
and  opens  G,  and  so  enters  the  receiver,  V. 
If  the  piston  be  raised,  air  rushes  in  at  F 
to  fill  the  space  in  the  cylinder.  It  cannot 
come  from  V,  because  the  valve  G  is  shut 
by  the  pressure  of  the  air  within.  By  work- 
ing the  piston  for  some  time,  you  can  force  a 
quantity  of  air  into  V  of  very  great  density. 
It  is  evident  that  this  instrument  is  the  very 
opposite  of  the  air-pump.  The  receiver,  V, 
contains  condensed  air,  while  the  receiver 
of  the  air-pump  contains  rarefied  air.  If 
you  compare  the  two  instruments,  you  will 
see  that  the  opposite  results  are  owing  to 
a  different  arrangement  of  the  valves. 
Until  quite  recently  air  had  never  been  condensed  to 
the  liquid  state.  This  was  accomplished  by  Messrs.  Pictet 
and  Cailletet,  who  subjected  it  to  enormous  pressure  and  a 
very  low  temperature.  The  term  permanent  gas  formerly 
applied  to  air  must  now  be  abandoned. 

The  elasticity  of  the  air  and  other  gases  results  from  an 
incessant  commotion  of  their  particles.  We  must  picture 
to  our  minds  the  molecules  of  a  gas  as  moving  in  all  direc- 
tions, constantly  striking  against  each  other,  and  thus  pro- 
ducing pressure  on  the  sides  of  an  enclosing  vessel.  We 
have  already  referred  to  this  motion  of  the  molecules  of  a 
gas  in  §  8.  The  force  with  which  the  molecules  strike 
against  the  confining  walls  will  be  greater  the  smaller  the 
space  through  which  they  are  allowed  to  move,  a  consider- 
ation which  explains  the  fundamental  principle  known  as 
Marriotte's  law — viz.,  the  pressure  of  any  quantity  of  gas 
is  inversely  proportional  to  its  volume.  That  is  to  say,  the 
greater  the  pressure  to  which  a  gas  is  subjected,  the  less 


PNEUMATICS.  221 

space  it  occupies.  Thus  a  body  of  air  which  under  a  cer- 
tain pressure  occupies  six  cubic  feet  will  be  condensed  to 
three  cubic  feet  by  twice  the  pressure,  and  to  two  cubic 
feet  by  three  times  the  pressure,  etc. 

Illustrations. — Air-guns  and  pop-guns  illustrate  the  elasticity  of  con- 
densed air.  The  air-gun  is  constructed  in  this  way :  A  receiver  like  V, 
Fig.  191,  is  so  made  that  you  can  screw  it  on  and  off  the  instrument. 
After  being  charged  with  condensed  air,  it  is  screwed  upon  the  gun,  its 
stem  communicating  with  the  barrel.  In  order  to  discharge  the  gun 
there  is  a  contrivance  connected  with  the  trigger  for  raising  the  valve, 
G,  so  that  some  of  the  condensed  air  may  enter  the  barrel.  On  doing 
so,  its  sudden  expansion  rapidly  forces  out  the  contents.  The  principle 
on  which  the  common  pop-gun  operates  is  similar.  Air  is  confined  be- 
tween the  two  corks,  P  and  P',  Fig.  192.  As  the  rod,  S,  is  pushed  quickly 


Fig.  192. 

in,  the  cork  P'  is  carried  nearer  to  P,  so  that  the  air  between  them  is 
condensed.  With  the  condensation  the  expansive  force  is  increased  ;  and 
when  it  becomes  so  great  that  the  cork  P  can  no  longer  resist  it,  it  throws 
the  cork  out,  and  so  quickly  as  to  occasion  the  popping  sound. 

The  explosion  of  powder  furnishes  a  good  illustration  of  the  expansive 
force  of  condensed  air  or  gases.  These  gases  are  produced  so  suddenly 
from  the  powder  that  at  the  instant  they  are  in  n  very  condensed  state, 
and  therefore  expand  powerfully.  The  power  of  steam  is  in  proportion  to 
its  condensation.  When  formed  under  the  confinement  of  a  boiler,  on 
being  allowed  to  escape  it  expands  with  great  force.  The  application  of 
the  expansive  power  of  steam  will  be  treated  of  particularly  in  §  182. 

139.  Pressure  of  the  Air  on  Liquids.  —  If  you  plunge  a 
tumbler  into  a  vessel  of  water,  and,  turning  it  over,  hold 
it  so  that  its  open  part  is  just  under  the  surface,  it  will  re- 
main full.  This  is  because  the  weight  of  the  air  pressing 
upon  the  surface  of  the  water  in  the  vessel  prevents  the 
water  in  the  tumbler  from  passing  downward.  Now, 

K 


222 


NATURAL   PHILOSOPHY. 


Fig. 193. 


if  you  introduce  a  bent 
tube  under  the  tumbler,  as 
shown  in  Fig.  1 93,  and  blow 
through  it,  the  air  forced 
up  into  the  tumbler  presses 
the  water  down,  taking  its 
place.  That  is,  the  press- 
ure of  the  air  within  the 
tumbler  acts  in  opposition 
to  the  pressure  of  the  air 
upon  the  surface  of  the 
water.  If  instead  of  a  tum- 
bler you  take  a  tall  jar,  as 
represented  in  Fig.  194,  and,  filling-it  with  water,  invert  it 
upon  a  small  shelf  placed  beneath  the 
surface  of  the  water,  you  will  have  a 
representation  of  the  pneumatic  trough 
used  by  the  chemist  in  collecting  gases. 
To  fill  the  jar  a  with  gas  he  puts  be- 
neath it  the  mouth  of  the  retort  from 
which  the  gas  issues,  and  the  gas  pass-  Fig.  194. 

ing  upward  expels  the  water.  In 
Fig.  195  is  represented  an  experiment 
which  shows  not  only  that  the  press- 
ure of  the  air  sustains  the  column  of 
water  in  the  cases  cited  above,  but 
also  that  it  makes  no  difference  in 
what  direction  this  pressure  is  exert- 
ed. Take  a  glass,  fill  it  even  full  with 
water,  and,  placing  a  piece  of  writing- 
paper  over  its  mouth,  carefully  invert 
it,  as  shown  in  the  figure.  The  paper 
will  remain,  and  the  water  will  not 
Fi-^.195.  run  out.  It  is  the  pressure  of  the  air 


PNEUMATICS.  223 

that  sustains  the  water,  and  the  paper  only  serves  to  main- 
tain the  surface  of  the  water  unbroken.  If  the  paper  were 
not  there  the  particles  of  the  air  would  insinuate  them- 
selves among  those  of  the  water,  and  pass  upward  in  the 
glass.  This  explains  why  a  liquid  will  not  run  from  a 
barrel  when  it  is  tapped,  if  there  be  no  vent-hole  above, 
unless  so  large  an  opening  be  made  as  to  let  the  air  work 
its  way  in  bubbles  among  portions  of  the  liquid.  It  is  this 
entrance  of  the  air  that  causes  the  gurgling  sound  heard 
in  pouring  a  liquid  from  a  bottle. 

140.  Amount  of  Atmospheric  Pressure. — If,  instead  of  the 
glass  jar  in  Fig.  194,  you  use  a  tube  thirty-four  feet  long,  and 
closed  at  the  top,  it  will  remain  full  of  water.     If  the  tube 
be  longer,  the  water  will  stand  only  at  thirty-four  feet,  leav- 
ing an  empty  space,  or  vacuum,  above  it.     It  makes  no  dif- 
ference what  the  size  of  the  tube  is ;  the  result  will  be  the 
same  in  all  cases.*     That  is,  a  column  of  water  thirty-four 
feet  high  can  be  sustained  by  the  pressure  of  the  atmos- 
phere.   It  is  easy,  therefore,  to  estimate  the  weight  or  press- 
ure of  the  air.    The  pressure  of  the  column  of  water  is  found 
to  be  fifteen  pounds  to  the  square  inch  of  its  base,  and  this, 
of  course,  is  the  amount  of  pressure  or  weight  of  the  atmos- 
phere which  it  balances.     Mercury  is  thirteen  and  a  half 
times  as  heavy  as  water,  and  therefore  the  air  will  sustain 
a  column  of  it  only  about  thirty  inches  in  height  (76  cm.). 

141.  Barometer. — The  weight  of  the  atmosphere  varies 
to  some  extent  at  different  times,  and  the  barometer  is 
an  instrument  for  measuring  these  variations.     It  is  con- 
structed on  the  principles  developed  in  the  previous  para- 
graphs.    Fig.  196,  on  the  following  page,  represents  a  very 
simple  form  of  the  instrument.    A  glass  tube  about  35  inches 


*  This  is  true  except  when  the  tube  is  so  small  that  capillary  attraction 
exerts  considerable  influence. 


224 


NATURAL   PHILOSOPHY. 


(88.8  centimetres)  long,  closed  at  one  end,  is  fill- 
ed  with  mercury,  and  then  inverted  in  a  cup  of 
the  same  liquid,  n  n.  The  vacuum 
produced  by  the  falling  of  the  mer- 
cury is  called  the  Torricellian  vacu- 
um, from  Torricelli,  an  Italian,  who 
first  developed  the  principles  of 
the  instrument  in  1642.  Fig.  197 
shows  another  form  of  the  instru- 
ment, with  a  scale  attached.  The 
mercury  generally  stands  at  the 
height  of  about  30  inches.  But  it 
varies  with  the  weather.  When 
the  weather  is  bright  and  clear, 
the  air  is  heavier,  and,  pressing 
upon  the  mercury  in  the  vessel, 
forces  it  up  higher  in  the  tube. 
But  when  a  storm  approaches,  the 
air  is  apt  to  be  lighter,  and  there- 
fore,  pressing  less  strongly  on  the 
mercury  in  the  vessel,  the  mercury 
in  the  tube  falls. 

The  barometer  is  of  great  service,  espe- 
cially at  sea,  in  affording  the  sailor  warning 
of  an  approaching  storm.  An  incident  is 
related  by  Dr.  Arnott  which  strikingly  illus- 


Fig.  196. 


Fig.l9T 

trates  its  value  in  this  respect.  He  was  at  sea  in  a  southern  latitude. 
As  the  sun  set  after  a  beautiful  afternoon  the  captain  foresaw  danger, 
although  the  weather  was  perfectly  calm,  for  the  mercury  in  the  barom- 
eter had  suddenly  fallen  to  a  remarkable  degree.  He  gave  hurried  or- 
ders to  the  wondering  sailors  to  prepare  the  ship  for  a  storm.  Scarcely 
had  the  preparations  been  made  when  a  tremendous  hurricane  burst  upon 
the  ship,  tearing  the  furled  sails  to  tatters,  and  disabling  the  masts  and 
yards.  If  the  barometer  had  not  been  observed,  the  ship  would  have 
been  wholly  unprepared,  and  shipwreck,  with  the  loss  of  all  on  board, 
would  in  all  probability  have  resulted. 


PNEUMATICS. 


225- 


A  water-barometer  could  be  made,  but  it  would  be  very 
unwieldy,  for  the  tube  must  needs  be  more  than  34  feet 
long.  Besides,  it  would  not  answer  in  very  cold  weather, 
for  the  water  would  freeze.  So  short  a  column  of  the  heavy 
fluid  mercury  balances  the  weight  of  the  atmosphere  that 
a  barometer  made  with  this  is  of  very  convenient  size  ;  and 
then  there  is  no  danger  of  the  mercury's  freezing,  except  in 
the  extreme  cold  of  the  arctic  regions. 

The  Barometer  a  Measurer  of  Heights. — The  atmosphere, 
as  stated  in  §  131,  diminishes  regularly  in  density  as  we  go 
upward.  The  rate  of  this  diminution  has  been  accurately 
ascertained,  and  therefore  we  can  estimate  heights  by  the 
amount  of  pressure  on  the  mercury  in  the  barometer.  At 
a  height  of  500  feet  the  barometer  will  be  half  an  inch 
lower  than  in  the  valley  below.  At  the  summit  of  Mont 
Blanc  it  stands  but  half  as  high  as  at  its  foot,  indicating  a 
height  of  15,000  feet.  Du  Luc,  in  his  famous  balloon  ascen- 
sion from  Paris,  saw  the  barometer  at  one  time  standing  at 
about  twelve  inches,  showing  an  elevation  of  21,000  feet. 

The  Aneroid  Barometer. — The  inconvenience  of  travel- 
ling in  mountainous  regions  with  a  long  tube  filled  with 
mercury  is  very  great,  and 
has  led  to  the  invention  of 
another  form  of  barometer 
which  is  called  an  Aneroid. 
The  principle  involved  in 
its  construction  may  be  ex- 
plained by  reference  to  Fig. 
198.  The  curved  tube  a  £, 
when  exhausted  of  air  and 
hermetically  closed,  is  sen- 
sitive to  the  variations  in 
the  pressure  of  the  atmos- 
phere, the  ends  of  the  tube  Fig.  19s. 


226 


NATURAL   PHILOSOPHY. 


approaching  with  increased  pressure,  and  receding  with  re« 
duced  pressure. 

Kow,  if  a  similar  tube  be  inserted  in  a  case,  and  the 
curved  ends  be  connected  by  means  of  a  mechanical  contriv- 
ance with  a  hand  like 
that  of  a  watch,  we  will 
have  the  simplest  possi- 
ble form  of  the  aneroid 
barometer.  The  hand 
points  to  figures  around 
the  dial-plate  of  the  in- 
strument corresponding 
to  the  height  of  the  mer- 
curial barometer.  The 
general  appearance  of 
such  an  instrument  is 
that  of  a  watch,  and  it 
is  but  little  larger  (Fig. 
199). 

142.  Relation  of  the  Air's  Pressure  to  the  Boiling-point. 
— Water  heated  to  212  degrees  Fahrenheit  (100°  Centi- 
grade) boils  —  that  is,  it  becomes  vapor.  If  water  bo 
heated  on  the  summit  of  a  high  mountain,  it  boils  be- 
fore it  reaches  this  temperature.  On  the  top  of  Mont 
Blanc  it  boils  at  180  degrees  (82.2°  C. )  —  that  is,  32 
degrees  (17.8°  C. )  below  the  boiling-point  of  water  at 
the  foot  of  the  mountain.  This  is  because  the  pressure 
of  the  air  acts  in  opposition  to  the  change  of  water  into 
vapor ;  and  the  less  the  pressure,  the  less  heat  will  be  re- 
quired to  vaporize  the  water.  We  may  illustrate  this  in- 
fluence of  the  pressure  of  air  upon  boiling  by  the  follow- 
ing experiment.  Let  a  cup  of  ether,  which  boils  at  95 
degrees  (35°  C.),  be  placed  under  the  receiver  of  an  air- 
pump.  On  rarefying  the  air  by  the  pump,  the  ether  will 


Fig.  199. 


PNEUMATICS. 


227 


boil.  The  general  - 
effect  of  pressure 
upon  boiling  may 
be  prettily  illustrat- 
ed by  another  ex- 
periment. Boil  some 
water  in  a  thin  flask 
over  a  spirit-lamp. 
While  the  steam  is 
still  issuing  cork  the 
flask  tightly,  invert 
it,  and  let  the  boiling 
cease.  If,  now,  you 
pour  some  cold  wa- 
ter over  the  flask, 
the  boiling  will  com- 
mence again  with 
considerable  energy. 
Why?  Because  you  condense  the  steam  above  the  water 
by  the  application  of  cold,  and  thus  remove  the  pressure. 
Then,  again,  if  you  pour  hot  water  over  the  flask  while  the 
water  is  boiling,  the  boiling  ceases,  because  the  heat  favors 
the  accumulation  of  steam,  and  therefore  renews  the  press- 
ure on  the  surface  of  the  water. 

It  is  evident  from  what  has  been  stated  that  most  liquids  have  that 
form  owing  to  the  pressure  of  the  atmosphere  upon*  them.  If  there  were 
no  atmosphere,  ether,  alcohol,  the  volatile  oils,  and  even  water,  would  fly 
off  in  vapor ;  and  the  earth  would  be  enveloped  in  a  gaseous  robe,  for  the 
particles  of  the  vapors  would  be  held  to  the  earth  by  attraction,  just  as  the 
particles  of  the  air  now  are. 

143.  Siphon. — The  pressure  of  air  upon  fluids  is  beauti- 
fully exemplified  in  the  operation  of  the  siphon.  This  in- 
strument is  simply  a  bent  tube  having  one  branch  longer 


Fig.  200. 


228 


NATUKAL   PHILOSOPHY. 


Fig.  201. 


than  the  other.  Its  operation 
is  shown  in  Fig.  201.  The  tube 
having  been  first  filled  with 
the  liquid,  its  shorter  branch  is 
placed  in  the  liquid  of  the  ves- 
sel A,  which  is  to  be  emptied, 
and  beneath  the  other  is  held 
the  vessel  B,  which  is  to  receive 
the  liquid.  As  shown  here,  the 
opening  of  the  long  branch  is 
below  the  surface  of  the  liquid 
in  B.  It  is  manifest,  therefore, 
that  the  air  presses  equally  upon  the  surfaces  in  both 
vessels,  tending  to  support  the  fluid  in  the  tube,  just  as 
the  water  is  supported  in  the  jar  in  Fig.  194.  But,  not- 
withstanding these  equal  pressures,  the  liquid  runs  up  the 
tube  from  A,  and  down  its  longer  branch  into  B.  Why 
is  this?  Since  the  pressure  of  a  column  of  fluid  is  in  pro- 
portion to  its  height,  there  is  greater  pressure  or  weight 
in  the  longer  branch  than  in  the  other;  and  it  is  this 
difference  in  wreight  that  causes  the  flow  from  A  into 
B  through  the  siphon.  The  difference  in  the  columns 
in  the  two  branches  is  not  the  difference  in  length  of 
these  branches,  but  the  distance  between  the  levels  of 
the  fluid  in  A  and  B  —  that  is,  the  distance  from  a  to  b. 
The  operation,  then,  of  the  instrument  is  this :  there  is  a 
constant  tendency  to  a  vacuum  at  C,  the  bend  of  the  tube, 
from  the  influence  of  gravitation  on  the  excess  of  fluid  in 
the  long  branch  over  that  in  the  short  one.  This  tendency 
is  constantly  counteracted  by  the  rise  of  fluid  in  the  short 
branch,  it  being  forced  up  by  the  pressure  of  the  air  upon 
the  surface  of  the  fluid  in  A. 

If  the  siphon  were  so  placed  that  the  surface  of  the 
liquid  in  A  were  precisely  on  a  level  with  that  in  B,  as  repre- 


PNEUMATICS. 


229 


sented  in  Fig.  202,  the  liquid 
would  remain  at  rest,  for, 
since  pressure  is  in  propor- 
tion to  the  height,  and  the 
pressures  on  the  two  sur- 
faces are  equal,  there  would 
be  an  exact  balance.  But  Fig.  202. 

let  the  surface  in  B  be  ever  so  little  lower  than  in  A,  and 
the  flow  will  begin.  And  the  greater  the  distance  between 
the  two  levels,  the  more  rapid  will 
be  the  flow,  for  the  greater  will  be 
the  influence  of  gravitation  in  the 
long  branch. 

Again,  if  the  end  of  the  long 
branch  of  the  siphon  be  free,  as  in 
Fig.  203,  the  siphon  will  operate  in 
the  same  way,  for  the  air,  pressing 
in  all  directions  equally,  tends  to 
support  the  column  of  fluid  in  the 
long  branch  by  a  direct  upward 
pressure,  but  is  prevented  from  do- 
ing so  by  the  excess  of  fluid  in  it 
above  that  in  the  shorter  one.  The 

operation  of  the  siphon  is  commonly  represented  in  this 
way;   but  we   have  given  first  the  ar- 
rangement  in  Fig.  202,  in  order  that  you 
might  more  clearly  see  the  principle  of 
the  instrument. 

Uses  of  the  Sip/ton. — The  siphon  is  used  chiefly 
for  discharging  liquids  from  one  barrel  or  vessel  into 
another.  For  convenience,  it  is  often  constructed 
after  the  plan  of  Fig.  204.  To  the  long  branch, 
B  C,  is  attached  the  tube  E  D.  It  is  used  in  this 
way :  The  end  of  the  short  branch,  A,  being  intro- 
duced into  the  liquid  to  be  drawn  off,  you  close  the 

K2 


Fig. 204. 


230 


NATURAL   PHILOSOPHY. 


run 


end  C  with  a  cork  or  your  finger;  and  after  filling  the  siphon  by  suction 
at  E,  you  remove  the  finger  and  let  the  liquid  run.  The  siphon  has 
sometimes  been  used  to  drain  pits  and  mines.  It  of  course  can  never  be 
used  where  the .  elevation  over  which  the  tube  is  to  bend  is  over  34  feet 
from  the  surface  of  the  water  to  be  discharged,  for  then  the  air  would 
not  press  the  water  up  to  the  bend  of  the  siphon. 

The  so-called  cup  of  Tantalus  is  a  pretty 
toy ;  it  consists  simply  of  a  goblet  contain- 
ing a  siphon  which  is  concealed  by  a  human 
figure.     In  Fig.  205  the  figure  is  omitted  to 
show  the  position  of  the  siphon.     On  pour- 
ing water  into  the  cup,  it  will  remain  there 
until  you  pour  in  enough  to  cover  the  bend 
of  the  siphon ;  as  soon  as  this  is  done,  the 
siphon  fills,  and  the  water  flows  out  through 
the  long  branch  which  passes  through  the 
bottom  of  the  cup.     The  lips  of  the  human 
Fig.  205.          figure  being  on  a  level  with  the  bend  of  the 
siphon,  it  is  apparently  prevented  from  drinking  in  a  tan- 
talizing way. 

144.  Intermitting  Springs. — The  operation  of  an  intermit- 
ting spring  is  essentially  the  same  with  that  of  the  cup  of 
Tantalus.  Fig.  206 
represents  such  a 
spring.  There  is  a 
cavity  in  a  hill,  sup- 
plied with  water  from 
a  source  above.  There 
is  also  a  passage  from 
the  cavity  which 
takes  a  bend  upward 
like  a  siphon.  Now,  when  the  water  in  the  cavity  is 
low,  it  will  not  run  out  from  the  siphon  -  like  channel ; 
but  when  the  cavity  becomes  filled  above  the  level  of 
the  bend,  the  water  will  at  once  flow  out,  just  as  it  does 


PNEUMATICS. 


231 


from  the  cup  of  Tantalus  as  soon  as  the  bend  of  its  siphon 
is  covered. 

145.  Pumps. — The  accompanying  cut  represents  a  com- 
mon form  of  pump.    A  tube  extends  down  into  the  well,  B. 
Above  this  is  the  barrel 
of  the  pump,  a  5,  in  which 
the  piston  works  up  and 
down.    There  is  a  valve 
in  the  piston,  and  anoth- 
er at  the  bottom  of  the 
barrel.      Both    of   them 
open  upward.    We  will 
suppose  that  the  pump 
is  entirely  empty  of  wa- 
ter.    If  the    piston    de- 
scend, the   piston   valve 
shuts  down,  and  the  low- 
er   valve    opens,  letting 
the  air  between  pass  up- 
ward.    When  the  piston 
rises,  the  air  above  the 
piston  cannot  get  below, 
for  its  pressure  will  shut 
the    valve    in    the    pis- 
ton.    But  there  will  be 
a  tendency  to  a  vacuum 
below   the   piston    as    it 
rises,  and    the    air    will 
pass    up    through    the 
valve    in    the   barrel    to 
fill  up   the   space.     But 
why  does  the   air  rise?  Fis-20L 

Because  of  the  pressure  of  the  air  upon  the  surface  of  the 
water  in  the  well.     This  forces  up  in  the  pump  the  water 


232 


NATURAL  PHILOSOPHY. 


and  the  air  above  it,  just  in  proportion  as  the  downward 
pressure  in  the  pump  is  lessened.  If  the  pumping  be  con- 
tinued, all  the  air  will  soon  be  expelled,  the  water  follow- 
ing it  and  flowing  out  at  the  opening,  r.  It  is  obvious 
that  the  pump  will  be  useless  if  the  valve  in  the  barrel  be 
over  34  feet  above  the  surface  of  the  water  in  the  well,  be- 
cause the  pressure  of  the  atmosphere  will  not  sustain  a 
higher  column  of  water. 

In  common  language,  the  operation  of  the  pump  is  attributed  to  what  is 
called  a  principle  of  suction,  as  if  there  were  a  drawing-up  of  the  water. 
But  that  water,  you  see,  is  not  drawn,  but  forced  up.  So  it  is  with  all 
operations  of  a  similar  character.  When  we  apply  the  mouth  to  suck  up  a 
fluid  through  a  tube,  the  fluid  is  forced  up  because  the  pressure  downward 
in  the  tube  is  removed.  But  how  is  it  removed  ?  It  is  done  by  a  move- 
ment of  the  tongue  downward  from  the  roof  of  the  mouth  ;  thus  removing 
the  pressure  of  the  air,  in  the  same  manner  as  the  upward  movement  of 
the  piston  in  the  pump.  To  fill  the  space  made  by  the  movement  of  the 
tongue,  the  air  is  forced  up  the  tube,  the  liquid  following ;  and,  as  in  the 
case  of  the  pump  when  the  air  is  all  expelled,  the  liquid  will  begin  to  dis- 
charge into  tke  mouth. 

Forcing-Pump. — The  forcing-pump  is  constructed  differ- 
ently from  the  common  pump.  Its  plan 
F  is  given  in  Fig.  208.  It  has  a  pipe,  C  D, 
and  a  barrel,  A  B,  like  the  common 
pump.  It  has  also  the  valve  E  at  the 
bottom  of  the  barrel.  But  it  has  no 
valve  in  the  piston.  Connected  with 
the  barrel  is  another  pipe,  F  G,  from 
which  the  water  issues.  This  has  a 
valve,  H,  opening  upward.  The  opera-, 
tion  of  the  pump  is  obvious.  As  the 
piston  is  drawn  up,  E  opens  and  H  shuts; 
and  when  it  is  forced  down,  E  shuts  and 
H  opens. 

146.  Fire-Engine. —  The  fire-engine  has  commonly  two 


PNEUMATICS. 


233 


forcing- pumps,  with  a  contrivance  for  making  the  water 
issue  in  a  uniform  stream.  This  contrivance  can  be  ex- 
plained by  reference  to  Fig.  209.  The  discharging -pipe, 
/*  #,  extends  down  into  a  large  vessel,  a,  which  is  filled 
with  air.  The  uniformity  of  the  stream  depends  upon  the 
elastic  force  of  compressed  air,  as  will  appear  from  an  ex- 
planation of  the  operation  of  the  machine.  When  the  wa- 


Fig. 209. 

ter  is  forced  through  the  openings  c  b,  it  compresses  the 
air  in  «,  for  the  tube  h  g  is  too  small  to  allow  all  the  water 
to  escape  that  comes  from  the  larger  tubes,  b  b.  Now,  the 
moment  that  the  piston  is  raised  it  ceases  to  force  the  wa- 
ter through  c,  and  the  elastic  force  of  the  compressed  air 
operates,  shutting  down  the  valve  c  and  forcing  the  water 
up  h  g.  The  result  is  a  continuous  rise  of  the  water  in 
this  tube,  and  therefore  a  uniform  stream.  The  valves  d  d 
permit  the  water  in  the  reservoir  surrounding  the  cylin- 
ders to  enter  when  the  pressure  in  e  is  relieved.  By  hav- 
ing two  cylinders  and  pistons  communicating  wfth  one  air- 


234  NATURAL  PHILOSOPHY. 

chamber,  «,  as  in  the  figure,  the  continuity  of  the  stream 
of  water  is  doubly  insured. 


QUESTIONS. 

128.  What  does  pneumatics  teach?  How  can  you  show  that  air  is  ma- 
terial ?  How  that  it  has  weight  ?  Describe  the  experiment.  What  is  its 
weight  compared  with  that  of  water? — 129.  What  is  said  of  the  attrac- 
tion of  the  air  by  the  earth  ?  Explain  why  some  things  rise  and  others 
fall  in  air.  — 130.  How  thick  is  the  earth's  air -covering?  How  is  the 
height  of  the  atmosphere  ascertained  ?  At  what  rate  does  the  earth  move 
round  the  sun? — 131.  State  the  influence  which  gravitation  has  upon  the 
density  of  the  air  at  different  heights.  Give  the  comparison  of  air  to  wool. 
What  is  said  of  hydrogen  and  balloons? — 132.  In  what  are  gases  and  liq- 
uids alike,  and  what  are  the  results  of  the  similarity? — 133.  What  is  the 
amount  of  pressure  of  the  atmosphere  on  each  square  inch  of  surface  ? 
Give  the  calculations  in  regard  to  this  pressure.  Show  why  the  great 
pressure  of  the  air  does  not  produce  injurious  effects.  —  1 34.  Describe 
the  air-pump.  Explain  by  Fig.  178  the  plan  and  working  of  the  air-pump. 
— 135.  State  some  of  the  experiments  with  the  air-pump.  How  can  you 
prove  that  air,  like  water,  presses  equally  in  all  directions  ?  State  the  com- 
parison about  the  fish.  What  is  said  of  the  Magdeburg  hemispheres  ?  Give 
the  experiment  with  mercury.  Explain  the  operation  of  the  boy's  sucker. 
Give  the  statements  about  sucker-like  arrangements  in  animals. — 136.  State 
the  experiment  of  the  bladder  and  weight.  Give  the  experiment  with  the 
India-rubber  bag.  State  the  experiment  with  the  egg.  Explain  the  opera- 
tion of  the  hydrostatic  balloon. — 137.  What  is  said  of  the  presence  of  air  in 
various  substances  ? — 138.  What  is  said  of  the  elasticity  of  air  ?  Describe 
and  explain  the  condenser.  What  is  meant  by  a  permanent  gas  ?  To  what 
is  the  elasticity  of  the  air  due  ?  What  is  Mnrriotte's  law  ?  Show  how  the 
air-gun  operates.  Explain  the  pop-gun.  Explain  the  operation  of  gun- 
powder. Explain  that  of  steam. — 139.  Describe  and  explain  what  is  rep- 
resented in  Fig.  193.  Explain  the  collection  of  gases  in  the  pneumatic 
trough.  Explain  the  experiment  represented  in  Fig.  194.  What  is  said 
of  tapping  a  barrel  ?  What  causes  the  gurgling  sound  when  a  liquid  is 
poured  from  a  bottle? — 140.  How  high  a  column  of  water  will  the  press- 
ure of  the  atmosphere  sustain  ?  How  do  you  find  from  this  the  pressure 
of  the  air  on  every  square  inch  of  surface?  How  high  a  column  of  mer- 
cury will  the  atmosphere  sustain? — 141.  Explain  the  barometer.  Relate 


SOUND.  235 

the  incident  given  by  Dr.  Arnott.  Why  would  not  a  water -barometer 
answer  ?  What  is  said  of  the  barometer  as  a  measurer  of  heights  ?  De- 
scribe the  aneroid  barometer.— 142.  How  is  the  boiling-point  influenced 
by  the  amount  of  the  air's  pressure?  Give  the  experiment  with  ether. 
State  the  experiment  with  the  flask.  What  would  happen  to  liquids  if 
the  atmosphere  were  removed  from  the  earth? — 143.  Explain  the  oper- 
ation of  the  siphon.  Explain  what  happens  if  the  siphon  be  placed  as 
shown  in  Fig.  203.  Explain  the  uses  of  the  siphon.  Explain  the  opera- 
tion of  the  cup  of  Tantalus. — 144.  How  are  intermitting  springs  account- 
ed for  ? — 145.  Explain  the  operation  of  the  common  pump.  Why  does  the 
water  rise  in  the  pump?  How  is  sucking  done?  Explain  the  forcing- 
pump. — 146.  Explain  the  working  of  a  fire-engine. 


CHAPTER  XIV. 

SOUND. 

147.  That  branch  of  natural  philosophy,  or  physics,  which 
treats  of  the  phenomena  of  sound  is  called  Acoustics,  the 
name  being  derived  from  a  Greek  word  meaning 
"  I  hear."    Acoustics  deals  mainly  with  the  produc- 
tion, transmission,  and  comparison  of  sounds,  leav- 
ing the  question  of  the  pleasurable  feelings  they 
may  arouse  to  the  science  of  music.   Sound  may  be 
defined  as  a  sensation  excited  in  the  organs  of  hear- 
ing resulting  from  the  vibratory  motion  of  bodies, 
which  motion  is  usually  transmitted  by  the  air. 

Bodies  which  emit  clear  and  regular  sounds  are 
said  to  be  sonorous.  That  the  production  of  sound 
is  due  to  their  vibrations  may  be  made  manifest  to 
the  senses  in  many  ways.  If  we  place  the  hand 
upon  a  large  bell  that  has  been  struck,  we  can  feel 
the  vibration.  If  we  strike  one  of  the  ends  of  a 
tuning-fork  upon  some  hard  body,  we  can  see  the 
vibration,  as  represented  in  Fig.  210  by  the  dotted  Fig.  210. 


236  NATURAL  PHILOGOPIIY. 

lines.  If  we  examine  the  strings  of  a  piano  while  it  is 
played,  the  vibration  of  the  larger  strings  is  very  notice- 
able. If  we  rub  the  edge  of  a  drinking-glass  with  a  moist- 
ened finger  so  as  to  produce  a  musical  sound,  the  water 
within  it  will  be  thrown  into  waves  by  the  vibration  of 
the  glass. 

In  wind  instruments,  as  the  flute,  horn,  etc.,  the  sound 
is  caused  by  the  vibration  of  the  body  of  air  within  the 
instrument.     In  the  common  tin  whistle  or 
bird-call,  Fig.  211,  the  sound  is  produced  by 
the  vibration  imparted  to  the  contained  air 
by  the  impulse  of  the  breath  through  the 
Fi°-211-         orifice,  B. 

148.  An  Analogy.— The  vibration  of  a  sonorous  body  is 
much  like  that  of  a  pendulum.  The  end  of  the  tuning-fork, 
Fig.  210,  on  being  struck  passes  to  #,  and  in  returning  pass- 
es by  the  point  of  rest,  A,  just  as  a  pendulum  does,  and 
reaches  a.  So,  also,  if  a  string,  tightly  stretched  between 
two  points,  A  B,  Fig.  212,  be  drawn  aside  to  D,  as  it  flies 

back  to  C  it  will  by  its  in-  n 

ertia  pass  on  to  E,  and  will    A<C',"^ 
continue  to  vibrate  back  and 
forth  for  some  time.      The 

same  rule  also  applies  to  the  extent  of  the  vibrations  here 
as  in  the  case  of  the  pendulum,  §  93.  The  quickness  of  the 
vibration  is  not  at  all  affected  by  its  width.  The  farther 
the  string,  A  B,  is  drawn  to  one  side,  the  greater  the  force 
with  which  it  will  return,  and  hence  it  will  reach  its  posi- 
tion on  the  other  side  of  the  middle  line  as  quickly  when 
drawn  far  away  from  this  line  as  it  would  if  drawn  but  a 
short  distance. 

The  vibrations,  however  produced,  are  transmitted  to  the 
ear  by  means  of  the  intervening  air.  The  latter  is  set  in 
motion  by  the  impact  of  the  vibrating  body,  much  as  mo- 


SOUND.  237 

tion  is  communicated  through  a  series  of  elastic  balls  (§  80). 
The  particles  of  air  swing  to  and  fro  through  a  short  dis- 
tance, being  condensed  at  one  point  and  thinner  at  an- 
other. A  succession  of  pulses  or  waves  ensues,  eacli  con- 
sisting of  a  pulse  of  condensation  and  a  pulse  of  rarefaction. 
The  to-and-fro  or  wave  motion  is  in  the  line  of  the  propa- 
gation of  the  sound.  The  manner  in  which  these  sound- 
pulses  act  upon  the  ear  will  be  explained  in  the  next  sec- 
tion. 

Only  a  limited  number  of  vibrations  produce  the  sensation  of  sound ; 
those  which  are  either  very  slow  or  very  quick  will  not  do  it.  Thus  if  a 
plate  of  metal  or  a  string  make  less  than  16  or  more  than  38,000  vibra- 
tions in  a  second,  no  effect  is  produced  upon  the  ear.  The  capacity  of 
hearing  differs,  however,  in  different  persons,  so  that  although  few  can  hear 
vibrations  which  are  beyond  the  range  mentioned,  there  are  many  whose 
capacity  falls  much  within  it  either  at  one  end  or  both  ends  of  the  scale. 
The  range  for  animals  is  not  the  same  as  that  for  man.  Thus  the  lion 
and  the  elephant  can  hear  a  sound  when  the  vibrations  are  too  infrequent 
to  make  any  impression  upon  our  ears ;  while  small  animals  have  a  sus- 
ceptibility in  the  organ  of  hearing  for  vibrations  so  rapid  that  we  cannot 
hear  them,  and  at  the  same  time  are  not  susceptible  to  the  slower  vibra- 
tions. How  far  the  range  varies  in  different  animals  has  not  been  ascer- 
tained to  any  extent. 

149.  How  the  Sensation  of  Sound  is  Produced. — The  vi- 
bration of  a  sounding  body  is  transmitted  to  the  ear  ordi- 
narily through  the  air,  and  there  strikes  upon  a  little  drum, 
a  membrane  at  the  bottom  of  the  external  cavity  of  the  ear 
somewhat  like  a  common  drum-head.  There  the  vibration 
of  the  air  is  communicated  to  this  drum,  and  from  this  to  a 
chain  of  very  small  bones.  From  the  last  of  these  bones  it 
is  transmitted  to  another  very  small  drum,  and  from  this  to 
a  fluid  in  some  very  complicated  passages  in  the  most  solid 
bone  in  the  body.  These  may  be  called  the  halls  of  audi- 
ence. In  the  fluid  contained  in  them  are  spread  out  the 
branches  of  the  nerve  of  hearing,  which  receive  the  impres- 


238 


NATURAL   PHILOSOPHY. 


sion  of  the  vibration,  and  transmit  it  to  the  brain,  where 
the  mind  takes  knowledge  of  it.  Observe  that  the  vibra- 
tion, transmitted  first  through  the  air,  then  through  the 
drum,  then  the  chain  of  bones,  then  another  drum  to  a 
fluid,  stops  at  the  fluid.  What  is  transmitted  from  this  to 
the  brain  by  the  nerve  we  know  not,  and  so  we  call  it  an 
impression. 

Sound  Transmitted  through  Various  Substances.  —  In  ordinary  hearing, 
sound,  as  you  have  seen,  is  transmitted  through  various  substances  before 
the  vibration  arrives  at  the  liquid  in  the  halls  of  audience.  But  sound 
need  not  take  this  course  in  all  cases  to  arrive  at  the  nerve  of  hearing.  If, 
for  example,  you  place  a  watch  between  your  teeth,  the  sound  will  go 
through  the  solid  teeth  and  the  bones  of  the  jaw  directly  to  the  halls  of 
audience  by  a  short  cut,  instead  of  going  round  through  the  outer  ear-pas- 
sage to  the  drum,  and  so  through  the  chain  of  bones.  Fishes  in  hearing 
receive  the  vibration  through  water.  If  you  place  your  ear  at  the  end  of  a 
timber,  while  some  one  scratches  with  a  pin  at  the  other  end,  you  hear  the 

sound  distinctly,  for  the  vibration  is 
transmitted  through  the  timber ;  as 
in  the  case  of  the  watch  between  the 
teeth,  it  goes  through  the  solid  bone. 

150.  Sound  not  Transmitted 
through  a  Vacuum. — As  sound 
is  a  vibration  of  some  sub- 
stance, it  cannot  be  transmitted 
through  empty  space.  This 
can  be  proved  by  an  experi- 
ment with  the  air-pump,  as  rep- 
resented in  Fig.  213.  Place  un- 
der the  receiver  a  clock-work 
furnished  with  a  bell  which 
can  be  made  to  ring  by  press- 
ing down  a  sliding  rod.  If  it 
be  struck  before  the  air  is  ex- 
Fig.  213.  hausted,  the  sound  is  heard 


SOUND.  239 

through  the  glass.  But  the  more  you  exhaust  the  air,  the 
fainter  will  be  the  sound;  and  at  length,  if  you  keep  on 
pumping,  it  cannot  be  heard  at  all.  A  similar  experiment 
can  be  tried  with  a  music-box.  It  is  owing  to  the  rarity 
of  the  air  on  high  mountains,  and  at  the  great  heights 
reached  by  balloons,  that  all  sounds  are  so  faint.  The  re- 
port of  a  pistol  fired  off  on  the  top  of  Mont  Blanc  is  no 
louder  than  the  snapping  of  a  whip,  and  trifling  compared 
with  its  report  when  fired  in  the  valley  below. 

151.  Sound  Caused  by  the  Resistance  of  the  Atmosphere. 
— Sound  is  often  heard  at  a  very  great  distance  on  the 
earth.     The  sound  of  an  eruption  of  a  volcano  has  been 
heard  in  one  case  at  the  distance  of  370  miles.     But  sup- 
pose that  the  same  sound  should  occur  at  the  same  dis- 
tance from  the  earth — that  is,  over  300  miles  beyond  the 
atmosphere  that  enrobes  the  earth — no  inhabitant  of  our 
world  could  hear  it,  for  the  same  reason  that  you  do  not 
hear  the  bell  ringing  in  an  exhausted  receiver.     If,  there- 
fore, nny  sound,  however  loud,  should  be  given  forth  by 
any  of  the  heavenly  bodies,  we  could  not  hear  it.     The 
course  of  these  bodies  in  their  orbits  is  noiseless,  because 
they  meet  with  no  resistance  from  any  substance.     Bodies 
passing  rapidly  through  our  atmosphere  cause  sound,  from 
the  resistance  which  the  air  gives  to  their  passage.     The 
whizzing  of  a  ball  is  an  example  of  this.     It  is  the  passage 
of  the  electric  fluid  through  the  air  which  produces  the 
thunder.     But  the  heavenly  bodies,  meeting  with  no  such 
resistance,  make  no  sound  in  their  course,  though  their  ve- 
locity be  so  immense.     In  the  expressive  language  of  the 
Bible,  "  their  voice  is  not  heard." 

152.  Velocity  of  Sound. — The  velocity  of  sound  varies  in 
different  media.     Thus  it  passes  through  water  four  times 
as  rapidly  as  through  air.    Dr.  Franklin,  having  placed  his 
head  under  water,  heard  distinctly  the  sound  of  two  stones 


240  NATUEAL   PHILOSOPHY. 

struck  together  in  the  water  at  the  distance  of  more  than 
half  a  mile.  Sound  passes  through  solids  much  more  eabi- 
ly,  and  therefore  more  rapidly,  than  through  liquids.  Thus 
its  velocity  through  copper  is  twelve  times  and  through 
glass  seventeen  times  greater  than  through  air.  If  you 
place  your  ear  against  a  long  brick  wall  at  one  end,  and 
let  some  one  strike  upon  the  other  end,  you  will  hear  two 
reports — the  first  through  the  wall  and  the  second  through 
the  air.  Indians  are  in  the  habit  of  ascertaining  the  ap- 
proach of  their  enemies  by  putting  the  ear  to  the  ground. 
When  the  eruption  of  a  volcano  is  heard  at  a  great  dis- 
tance, the  sound  comes  through  the  solid  earth  rather  than 
through  the  air.  The  ready  transmission  of  sound  through 
solids  furnishes  us  with  a  very  valuable  means  of  examin- 
ing diseases  of  the  lungs  and  heart.  The  sounds  occasioned 
by  the  movement  of  the  air  in  the  lungs  and  by  the  action 
of  the  heart  are  very  distinctly  heard  through  the  solid 
walls  of  the  chest. 

153.  Measurement  of  Distances  by  Sound.  —  Whether 
sound  be  loud  or  weak  makes  no  difference  in  its  veloc- 
ity. Thus  the  sounds  of  a  band  of  music  at  a  distance  all 
reach  your  ear  at  the  same  time,  the  sounds  of  the  instru- 
ments that  can  scarcely  be  heard  keeping  exact  pace  in 
the  air  with  the  sounds  of  the  loudest.  The  velocity  of 
sound  is  uniform  throughout  its  whole  course,  being  just 
as  rapid  when  it  is  about  to  die  away  as  it  was  when 
it  began.  This  uniformity  in  the  velocity  of  sound  en- 
ables us  to  estimate  the  distance  of  the  object  by  which 
any  sound  is  made.  We  do  it  by  a  comparison  between 
light  and  sound.  Sound  moves  at  the  rate  of  1120  feet  in 
a  second.  Now,  light  moves  192,000  miles  a  second,  and 
therefore,  for  all  ordinary  distances  on  the  earth,  we  need 
make  no  allowance  of  time  for  light  in  comparison  with 
sound.  If  we  see,  then,  the  operation  by  which  a  sound 


SOUND.  241 

is  produced,  we  can  estimate  its  distance  from  us  by  the 
length  of  time  which  elapses  between  what  we  see  and 
what  we  hear.  In  this  way  we  can  estimate  very  accurate- 
ly the  distance  of  a  cannon  that  we  see  fired,  or  the  dis- 
tance of  a  flash  of  lightning.  When  the  interval  between 
the  flash  and  the  peal  of  thunder  is  about  four  and  a  half 
seconds,  the  distance  of  the  cloud  is  about  one  mile. 

154.  Loudness  of  Sound. — The  loudness  of  sound  depends 
upon  the  width  of  the  vibrations  producing  it.    The  harder 
you  strike  the  end  of  the  tuning-fork,  Fig.  210,  the  farther 
will  it  vibrate  the  one  way  and  the  other,  and  the  louder 
will  be  the  sound.     The  same  thing  is  true  of  the  strings 
of  a  piano.     A  round  bell,  when  struck,  tends  in  its  vibra- 
tion to  take  an  oval  form,  and  the  extent  of  its  vibration 
back  and  forth  determines  the  loudness  of  the  sound.     As 
sound  passes  from  the  sounding  body  the  vibration  grad- 
ually lessens,  and  at  length  dies  away.     It  is  like  the  suc- 
cessive vibrations  or  waves  of  water  produced  by  drop- 
ping a  stone  in  it.     The  louder  the  sound,  the  larger  are 
the  first  vibrations  and  the  farther  will  the  vibrations  ex- 
tend ;  as  a  large  stone  dropped  into   water  will  produce 
larger  waves  than  a  small  one,  and  the  waves  will  extend 
over  a  greater  space. 

The  loudness  of  sound  decreases  very  rapidly  as  the  dis- 
tance from  its  source  increases;  at  twice  the  distance  the 
intensity  is  only  one  fourth ;  at  three  times  the  distance, 
one  ninth,  etc. ;  the  law  being  the  same  as  that  of  gravita- 
tion— viz.,  the  intensity  is  inversely  as  the  square  of  the 
distance.  (See  §  28.)  When  there  is  no  hindrance,  sound 
spreads  equally  in  all  directions.  In  this  respect  the  vibra- 
tions or  waves  of  air  resemble  waves  of  water.  Light  is 
also  diffused  in  the  same  manner,  as  you  will  learn  in 
another  chapter. 

155.  Reflection  of  Sound. — If  an  elastic  body  be  thrown 


242 


NATURAL   PHILOSOPHY. 


perpendicularly  upon  a  surface,  it  rebounds  in  the  same 
path  in  which  it  is  thrown.  But  if  it  hit  the  surface  ob- 
liquely, it  is  thrown  off  or  reflected  in  a  different  direction. 

Thus  a  ball  thrown  from 
p  upon  the  surface  s  s'  at 
the  point  n  will  rebound 
on  the  same  line,  np\  but 
if  it  be  thrown  from  /,  it 
will  be  reflected  in  the  di- 
rection n  d.  The  angle  i 
between  the  lines  fn  and 
n  p  is  called  the  angle  of 
incidence,  and  the  angle  r 
between  the  lines  p  n  and 
n  d  is  called  the  angle  of  reflection :  these  two  angles  are 
always  equal  if  the  body  projected  be  perfectly  elastic. 
Waves  of  sound  are  reflected  in  accordance  with  the  same 
law.  The  reflection  of  sound  is  the  cause  of  echoes.  In 
order  that  an  echo  be  perfect,  the  sound  must  be  reflected 
back  to  the  ear  from  a  plane  surface  of  some  size.  Some- 
times successive  plane  surfaces  of  rocks  in  a  valley,  or 
along  a  river,  cause  a  series  of  echoes.  Thus  in  Fig.  215 


Fiji.  214. 


Fig.  215. 


SOUND.  243 

is  represented  a  locality  on  the  Rhine  where  a  sound  is 
reflected  at  successive  places,  1,  2,  3,  4.  The  rolling  of 
thunder,  though  sometimes  caused  by  the  different  dis- 
tances of  parts  of  the  same  flash  of  lightning,  is  common- 
ly owing  to  reflections  of  the  sound  among  the  clouds. 
From  this  cause  the  report  of  a  cannon  is  more  apt  to  be  a 
rolling  sound  when  there  are  clouds  above  than  when  the 
sky  is  clear.  Sound  is  continually  reflected  in  every  varie- 
ty of  direction  from  obstacles  with  which  it  meets.  Thus 
in  a  room  it  is  reflected  from  the  walls  and  from  all  the 
objects  in  the  room ;  and  the  more  varied  are  the  surfaces, 
the  more  varied  and  confused  are  the  reflections.  You 
know  that  a  voice  has  a  very  different  sound  in  an  emp- 
ty room  from  that  which  it  has  when  the  room  is  filled 
with  an  audience.  Indeed,  a  blind  speaker  can  estimate 
very  nearly  the  size  of  his  audience  by  the  sound  of  his 
own  voice.  The  explanation  is,  that  with  a  full  audience 
the  surfaces  for  reflection  are  vastly  multiplied,  and  so  de- 
prive the  sound  of  the  sharp  and  ringing  character  which 
is  given  to  it  by  reflection  from  comparatively  few  surfaces 
which  are  plane  and  firm.  The  effect  produced  by  an  au- 
dience upon  the  voice  of  the  speaker  is  quite  analogous 
to  that  of  muffling  upon  the  sound  of  a  drum. 

156.  Whispering-Galleries. — The  reflection  of  sound  from  curved 
surfaces  gives  us  some  interesting  phenomena.  The  waves  of  sound  in  be- 
ing reflected  from  a  concave  surface  are  gathered  together  at  some  point. 
If  the  surface  be  a  perfectly  spherical  one,  and  the  sound  issue  from  the 
centre,  the  reflection  will  be  from  all 
points  to  the  centre.  But  suppose  the  _ 
concave  surface  have  the  curve  of  an 
ellipse,  as  represented  in  Fig.  216.  This, 
instead  of  having  a  centre,  has  two  foci, 
c  and  g.  Now  if  a  sound  proceed  from 

one  focus,  c,  the  waves  of  sound,  as  represented  by  the  lines  c  d,  c  e,  c  f, 
c  h,  will  all  be  reflected  to  the  other  focus,  y ;  so  that  if  a  person  speak  in  a 


244  NATURAL   PHILOSOPHY. 

very  low  tone  or  even  whisper  at  c,  he  may  be  heard  distinctly  by  another 
at  #,  though  persons  at  other  points  may  hear  nothing.  This  phenomenon 
may  occur  with  a  curved  wall  extending  even  several  hundred  feet;  and 
such  structures  are  called  whispering-galleries.  If  in  one  of  these  galleries 
a  person  standing  in  one  focus  speak  softly,  he  will  be  heard  by  others  at 
any  point  by  the  direct  waves  of  sound ;  but  the  reflected  sound  will  be 
added  to  the  direct  in  the  case  of  one  standing  at  the  other  focus.  The 
whispering-gallery  in  the  dome  of  St.  Paul's  Cathedral,  London,  is  a  cele- 
brated example  well  worth  visiting. 

157.  Concentration  of  Sound. — It  is  by  the  reflection  of 
sound  that  it  can  be  concentrated  in  various  ways.  Thus 
in  using  a  speaking-trumpet  the  waves  of  sound,  instead  of 
moving  in  all  directions  as  soon  as  they  escape  from  the 

mouth,  are  reflected 
by  the  sides  of  the 
instrument  towards 
a  central  line,  as  rep- 
resented in  Fig.  217. 

Fis-21T-  "        The  waves  or  vibra- 

tions, being  thus  concentrated,  have  more  intensity,  and 
are  thrown  to  a  greater  distance  than  if  they  issued  di- 
rectly from  the  mouth.  In  a  similar  manner  a  speaking- 
tube,  confining  the  vibrations,  carries  the  voice  to  distant 
parts  of  a  building.  For  the  same  reason  the  voice  can  be 
heard  much  farther  through  a  narrow  street  than  in  an 
open  space.  A  speaker  can  be  heard  more  distinctly  in 
a  hall  than  when  addressing  an  audience  of  the  same  size 
in  the  open  air.  The  "sounding-board,"  once  so  fash- 
ionable in  churches,  was  really  of  considerable  service  in 
preventing  the  escape  of  the  vibrations  of  the  voice  of 
the  preacher  upward,  and  directing  them 
downward  upon  the  audience.  In  the  hear- 
ing-trumpet, Fig.  218,  the  vibrations  are  col- 
lected in  the  broad  open  end  of  the  instru- 
ment and  by  reflection  are  thrown  together  Fig.2is. 


SOUND.  245 

into  a  narrow  compass-  before  they  enter  the  ear  to  strike 
upon  the  drum.  We  often  instinctively  make  the  palm  of 
the  hand  act  as  an  ear-trumpet  when  we  do  not  hear  dis- 
tinctly. Many  animals  have  the  external  ears  movable,  so 
that  they  can  direct  their  concave  surface  towards  the 
point  from  which  they  wish  to  hear.  Such  ears  act  like 
movable  ear-trumpets. 

158.  Difference  between  a  Musical  Sound  and  a  Noise. — 
The  difference  between  a  musical  sound  and  a  noise  is  very 
analogous  to  the  difference  between  a  crystal  and  the  same 
substance  destitute  of  the  crystalline  arrangement.  In 
both  sound  and  noise  there  are  vibrations,  but  in  music- 
al sound  they  recur  at  equal  intervals  of  time ;  while  in  a 
noise  the  vibrations  are  irregular,  and  there  is  confusion. 
Indeed,  so  regular  are  the  vibrations  of  musical  sounds  that 
the  rules  and  principles  of  music  have  all  the  rigid  exact- 
ness of  mathematics. 

Musical  sounds  differ  among  themselves  in  three  par- 
ticulars— loudness,  pitch,  and  quality.  The  loudness,  or  in- 
tensity, depends,  as  already  stated  (§  154),  upon  the  width 
of  the  vibrations  of  the  sounding  body.  The  pitch  depends 
upon  the  rapidity  of  these  vibrations;  the  quicker  the  vi- 
bration, the  higher  is  the  note.  Thus  a  short  and  small 
string  on  a  violin  or  in  a  piano  gives  a  higher  note  than  a 
long  and  large  string,  because  its  vibrations  are  quicker. 
The  tension  of  the  string  also  has  an  influence,  the  note 
being  raised  by  increasing  the  tension.  In  tuning  a  violin 
the  right  pitch  is  given  to  each  string  by  lessening  or  in- 
creasing the  tension  by  means  of  the  screws  to  which  the 
strings  are  attached.  In  playing  upon  it  various  notes  are 
made  upon  each  string  by  shortening  the  vibrating  portion 
more  or  less  by  pressure  of  the  finger. 

In  wind  instruments  the  note  depends  on  the  length  and 
size  of  the  column  of  air  contained  in  them.  This  may  be 

L 


246 


NATURAL   PHILOSOPHY. 


illustrated  by  an  organ-pipe,  Fig.  219.  It  is  one 
of  the  pipes  of  what  is  called  the  flute-stop.  It 
is  constructed  very  much  like  a  boy's  willow 
whistle.  The  air  from  the  bellows  of  the  organ 
enters  at  P,  and,  passing  through  the  narrow  slit, 
c  d,  is  projected  against  the  edge  of  the  mouth, 
a  #,  and  causes  a  vibration  of  the  whole  column 
of  air  in  the  pipe.  The  pitch  of  the  musical  note 
depends  in  part  on  the  length  of  the  column  of 
air  set  in  vibration,  and  in  part  on  the  size  of 
the  pipe. 

It  is  owing  to  the  difference  in  rapidity  of 
vibration  that  a  large  bell  gives  a  graver  note 
than  a  small  one.  When  musical  sounds  are 
produced  by  passing  the  moistened  fingers  over 
tbe  edges  of  glass  vessels,  the  larger  the  vessel, 
the  graver  its  note.  A  tumbler  will  give  a 
graver  note  than  a  wine-glass. 

The  third  peculiarity  of  musical  sounds,  called 
quality,  is  that  which  enables  us  to  distinguish 
the  notes  of  different  instruments  even  when 
their  pitch  is  the  same.  It  also  gives  the  dis- 
tinctive character  to  the  voices  of  different  animals,  and 
v3ven  of  different  persons.  It  depends  upon  the  form  of 
the  vibratory  motion, 
as  illustrated  by  A 
Fig.  220.  The  three 
waves,  A  B,  C  D,  and 
E  F,  there  represent- 


ed have  the  same 
width  and  the  same 
length,  and  conse- 


Fig.  219. 


\ 


\ 


\ 


quently  the  sounds  corresponding  would  have  the  same 
pitcli  and  intensity ;  but,  having  different/orms,  the  sounds 


SOUND.  247 

would  be  unlike  in  quality.  This  method  of  exhibiting  the 
subject  does  not  rest  on  theory  alone,  but  can  be  demon- 
strated to  the  eye  by  a  beautiful  experiment  of  a  delicate 
character  which  we  cannot  here  detail. 

159.  Human  Voice. — The  principles  developed  in  relation 
to  musical  instruments  apply  to  the  voice.  The  musical 
instrument  of  man,  by  which  the  voice  is  produced,  is  con- 
tained in  a  very  small  compass.  It  is  that  box  at  the  top 
of  the  throat  commonly  called  Adam's  apple.  Across  this, 
from  front  to  rear,  stretch  two  sheets  of  membrane,  leaving 
a  space  between  their  edges.  In  our  ordinary  breathing 
these  membranes  are  relaxed,  and  the  space  between  their 
edges  is  considerable,  to  allow  the  air  to  pass  in  and  out 
freely.  But  when  we  speak  or  sing  these  membranes,  or 
vocal  chords,  as  they  are  termed,  are  put  into  a  tense  state 
by  muscles  pulling  upon  them,  and  the  opening  between 
them  is  lessened.  The  voice  is  produced  by  the  air  that  is 
forced  out  from  the  lungs,  which,  striking  on  the  chords, 
causes  them  to  vibrate.  The  nearer  their  edges  are  to- 
gether, and  the  more  tense,  the  higher  the  note.  The 
sounds  are  produced  precisely  like  those  of  the  ^Eolian 
harp,  the  air  causing  in  the  one  case  a  vibration  of  strings, 
and  in  the  other  of  edges  of  membranes. 

160.  Harmony. — When  a  number  of  notes,  sounded  at  the  same 
time,  are  agreeable  to  the  ear,  they  are  said  to  harmonize.  Now  this  har- 
mony depends  on  a  certain  relation  between  the  vibrations.  The  more 
simple  the  relation,  the  greater  is  the  harmony.  For  example,  if  we  take 
the  first  note,  termed  the  fundamental  note,  of  what  is  called  the  scale  in 
music,  it  harmonizes  better  with  the  octave  than  with  any  other  of  the  eight 
notes,  because  for  every  vibration  in  it  there  are  just  two  in  the  octave. 
Take  in  contrast  with  the  octave  the  second  note.  Here  to  every  eight  vi- 
brations of  the  first  note  we  have  nine  of  the  second,  and  the  consequence 
is  a  discord  when  they  are  sounded  together.  The  difference  between  the 
two  cases  is  this :  In  the  first  case  the  commencement  of  every  vibration 
in  the  fundamental  note  coincides  with  the  commencement  of  every  second 


248  NATUKAL   PHILOSOPHY. 

vibration  in  the  octave.  But  in  the  other  case  there  is  a  coincidence  at 
only  every  eighth  vibration  of  the  first  note  with  every  ninth  of  the  second. 
Next  to  the  octave,  the  most  agreeable  harmony  with  the  fundamental  note 
is  that  of  the  fifth  note  of  the  scale.  Here  we  have  three  vibrations  to 
every  two  of  the  first  note,  and  so  every  second  vibration  in  the  first  note 
coincides  with  every  third  vibration  of  the  fifth.  Next  comes  the  harmony 
of  the  fourth,  there  being  here  a  coincidence  at  every  third  vibration  of  the 
fundamental  note.  The  more  frequent,  you  see,  are  the  coincidences  be- 
tween the  vibrations,  the  greater  is  the  harmony.  In  the  three  cases  just 
stated  the  coincidence  is  in  the  first  at  the  commencement  of  every  vibra- 
tion of  the  fundamental  note ;  in  the  second  case,  at  the  commencement 
of  every  second  vibration ;  and  in  the  third,  at  the  commencement  of  every 
third  vibration. 

161.  The  Gamut,  or  Diatonic  Scale.  — In  order  that  you  may 
see  the  relative  numbers  of  the  vibrations  for  each  of  the  notes,  we  give 
them  for  the  whole  scale.  They  are  as  follows  : 

i     I     I     3     i     f  .-¥-     2 

CDEFGABC 

According  to  this,  the  note  D  has  nine  vibrations  to  every  eight  vibrations 
of  C,  E  has  five  to  every  four  of  C,  etc.,  the  octave  C  having  just  twice  the 
number  of  vibrations  as  the  fundamental  note  C.  By  this  means  is  ex- 
pressed the  proportion  between  the  numbers  of  vibrations  in  the  different 
notes.  Suppose,  then,  that  you  know  the  number  of  vibrations  in  a  second 
required  for  C,  the  fundamental  note,  you  can  readily  calculate  the  number 
of  vibrations  of  each  of  the  other  notes.  It  is  done  by  multiplying  the  num- 
ber which  C  has  by  the  fractions  placed  over  the  other  notes.  Thus  if  the 
number  of  vibrations  in  a  second  in  the  fundamental  note  be  128,  by  this 
process  we  make  the  vibrations  of  all  the  notes  to  be  thus  : 

CDEFGABC 

128      144      1GO      170      192      213      240      256 

There  are  really  but  seven  notes  in  what  is  called  the  diatonic  scale,  the 
eighth  note,  C,  being  truly  the  first  of  seven  other  notes  above,  having  re- 
lations to  each  other  similar  to  those  of  the  notes  below,  and  constituting 
another  octave.  So  we  may  have  several  octaves,  one  above  another. 

It  is  interesting  to  observe  that  the  proportionate  lengths  of  strings  re- 
quired to  produce  the  eight  notes  of  the  scale  have  an  exact  numerical  re- 
lation, but  the  reverse  of  that  of  the  numbers  of  the  vibrations.  Thus  if 


SOUND.  249 


you  have  eight  strings  of  the  same  size,  their  vibrating  lengths  required  f 
the  notes  are  as  follows  : 

CDEFGABC 


For  the  notes  of  the  octave  above,  the  lengths  are  : 

CDEFGAB       C 
*       *       §       I       I      1%     &      £ 

162.  Unison.  —  In  tuning  instruments  so  as  to  make  them 
harmonize,  the  result  is  obtained  when  the  corresponding 
parts  of  the  instruments  have  the  same  number  of  vibra- 
tions.    Thus  the  string  in  one  violin  that  gives  any  partic- 
ular note  must  vibrate  just  the  same  number  of  times  in  a 
second  as  the  strings  giving  the  same  note  in  other  violins, 
or  it  will  not  be  in  perfect  unison  with  them.    The  same  is 
true  of  other  strings  for  other  notes,  and  also  of  the  corre- 
sponding parts  of  all  kinds  of  instruments.  which  are  to  be 
played  together.    When,  in  tuning  instruments  together,  it 
is  said  that  a  string  of  a  violin,  for  example,  is  too  flat,  the 
difficulty  is  that  it  does  not  vibrate  with  sufficient  rapidity, 
and  it  is  therefore  tightened  to  make  its  note  sharp  enough, 
as  it  is  expressed,  to  be  in  unison  with  the  note  of  the  cor- 
responding strings  or  parts  of  other  instruments. 

163.  Mysteries  of  Sound  and  Hearing.  —  There  are  many 
things  of  a  mysterious  character  in  relation  both  to  sound 
and  the  manner  in  which  it  causes  the  sensation  of  hear- 
ing.    We  will  barely  notice  but  two  of  these.     The  effect, 
or  rather  the  chain  of  effects,  resulting  in  hearing  is  whol- 
ly mechanical,  until  we   come   to   the   nerve  of  hearing, 
which  branches   out  with  minute  fibrils  in  the  halls  of 
audience  of  the  internal  ear.     It  is  merely  a  series  of  vibra- 
tions.    Now,  how  the  mere  agitation  of  a  fluid  enclosed 
in  hard  bone  can  communicate  through  fine  white  fibres 
to  the  brain,  and  through  that  to  the  mind,  the  impres- 
sion of  all  the  various  sounds  produced  is  a  great  mys- 


250  NATUKAT,   PHILOSOPHY. 

tery.  All  that  we  know  is  that  the  nerve  is  the  medium 
of  the  communication,  but  of  the  manner  in  which  it  per- 
forms its  office  we  know  absolutely  nothing.  Again,  while 
it  is  sufficiently  mysterious  that  this  information  can  thus 
be  given  to  the  mind  when  one  sound  after  another  com- 
municates its  vibration  to  the  liquid  in  the  ear,  the  mystery 
is  greatly  enhanced  when  various  sounds  come  to  the  ear 
at  one  and  the  same  time.  To  get  a  distinct  idea  of  the 
very  complex  and  wonderful  character  of  the  process  of 
hearing  in  such  a  case,  we  will  suppose  that  a  full  band  of 
music  is  playing,  and  at  the  same  time  mingled  with  its 
sounds  there  are  various  other  sounds  heard,  some  of  them 
perhaps  discordant.  What  a  diversity  of  vibrations  we 
have  here !  We  have  the  slow  vibrations  produced  in  the 
grave  notes,  and  the  quick  vibrations  of  the  higher  ones, 
all  travelling  together  through  the  air  to  the  ear,  and  each 
preserving  its  distinctive  character.  And  more  than  this, 
after  they  arrive  at  the  ear  they  are  communicated  unal- 
tered through  the  drum,  the  chain  of  bones,  the  second 
drum,  and  the  liquid  where  the  nerve  is,  so  that  a  correct 
report  of  each  of  all  the  notes  is  given  through  the  nerve 
to  the  mind.  Then,  too,  if  there  be  any  discord,  its  vibra- 
tion travels  along  with  the  rest,  and  so  do  the  vibrations 
of  other  sounds,  as  the  roaring  of  the  wind,  the  report  of 
cannon,  and  the  noise  of  the  people.  And  besides  all  this,  in 
the  multiplicity  of  the  vibrations  thus  transmitted  through 
so  many  different  substances  the  mind  gets  a  true  report 
of  the  comparative  loudness  of  the  sounds,  and  even  of  their 
character,  so  that  the  sounds  of  drum,  fife,  trumpet,  etc.,  are 
all  accurately  distinguished.  In  view  of  such  wonders,  how 
significant  is  the  question, "He  that  planted  the  ear,  shall 
he  not  hear?" 


SOUND.  251 

QUESTIONS. 

147.  What  is  the  meaning  of  Acoustics?  Define  sound.  Mention  cases 
in  which  the  vibration  of  sounding  bodies  is  manifest  to  the  sight  and 
touch.  What  is  said  of  wind  instruments? — 148.  State  the  analogy  of  a 
sounding  body  to  a  pendulum.  How  does  the  air  transmit  sound?  What 
is  the  connection  between  sound  and  rapidity  of  vibration? — 149.  Describe 
the  process  by  which  the  sensation  of  sound  is  produced.  Where  does  the 
vibration  caused  by  the  sounding  body  stop  in  the  ear  ?  What  is  trans- 
mitted thence  to  the  brain  ?  Give  examples  of  the  transmission  of  sound 
through  various  substances.  — 150.  State  the  experiment  by  which  it  is 
shown  that  sound  is  hot  transmitted  through  a  vacuum.  What  is  said  of 
sound  at  great  heights? — 151.  How  far  has  the  sound  of  a  volcano  been 
heard  ?  If  the  same  sound  were  made  in  space  at  that  distance  from  the 
earth,  why  could  not  the  inhabitants  hear  it?  What  is  the  cause  of  the 
noise  of  bodies  passing  through  the  air  ?  Why  do  the  heavenly  bodies, 
moving  so  rapidly,  produce  no  sound  ? — 152.  Cite  examples'  showing  the 
different  velocities  of  sound  in  different  media.  What  is  said  of  the  uni- 
formity of  the  velocity  of  sound? — 153.  Show  how  we  can  measure  dis- 
tances by  sound  as  compared  with  light  in  velocity. — 151.  Upon  what  does 
the  loudness  of  sound  depend  ?  Illustrate  this  point.  What  is  said  of  thci 
diffusion  of  sound  ?— 155.  What  of  its  reflection  ?  What  of  echoes  ?  What; 
is  said  of  multiplied  and  mingled  reflections  of  sound? — 150.  Explain  th<t 
operation  of  whispering-galleries  by  Fig.  217. — 157.  Explain  the  operation 
of  the  speaking-trumpet.  Give  other  examples  of  the  concentration  of 
sonorous  vibrations. — 158.  What  is  the  difference  between  a  musical  sound 
and  a  noise?  What  is  said  of  the  exact  regularity  of  musical  vibrations! 
Name  the  three  points  in  which  musical  sounds  differ.  Upon  what  does 
the  pitch  depend?  How  are  different  notes  produced  in  stringed  instru- 
ments ?  Upon  what  does  the  note  depend  in  wind  instruments  ?  Explain 
the  operation  of  the  organ-pipe.  What  is  said  of  the  notes  of  bells  and  of 
musical  glasses  ?  What  is  meant  by  quality  ?  Upon  what  does  it  depend  ? 
Illustrate  this. — 159.  Explain  the  mechanism  of  the  human  voice. — 1GO. 
What  is  harmony  ?  Upon  what  does  it  depend  ?  Between  what  two  notes 
of  the  scale  is  there  the  greatest  harmony  ?  What  note  next  to  the  octave 
harmonizes  best  with  the  fundamental  note?  And  what  note  next?  Show 
why  the  second  note,  in  contrast  with  the  octave,  is  so  discordant  with  the 
fundamental  note. — 161.  State  the  proportions  between  the  numbers  of  the 
vibrations  in  the  different  notes.  If  you  know  the  number  of  vibrations 
of  the  fundamental  note  in  a  second,  how  may  you  determine  the  number 


252  NATURAL   PHILOSOPHY. 

of  vibrations  in  the  other  notes?  What  is  said  of  the  numher  of  notes  in 
the  diatonic  scale  ?  What  of  the  proportionate  lengths  of  strings  for  dif- 
ferent notes? — 1G2.  What  is  said  of  tuning  instruments  ?  What  is  meant 
by  saying  that  a  note  is  too  sharp  or  too  flat? — 163.  State  in  full  what  is 
said  about  the  mysteries  of  sound  and  hearing. 


CHAPTER  XV. 

HEAT. 

164.  Heat  and  Cold. — In  common  language  we  speak  of 
heat  and  cold  as  two  distinct  and  opposite  things.  That 
this  is  not  strictly  correct  may  be  shown  by  the  following 
experiment :  Take  three  vessels,  and  fill  the  first  with  ice- 
cold  water,  the  second  with  hot  water,  and  the  third  with 
tepid  water.  If  you  place  your  right  hand  in  the  first  and 
the  left  in  the  second,  and  let  them  remain  a  little  time, 
on  taking  them  out  and  plunging  them  together  into  the 
third  vessel,  the  water  in  it  will  feel  warm  to  the  right 

'  O 

hand  and  cold  to  the  left.  Thus  the  air  of  a  cellar  seems 
warm  to  you  in  winter  and  cold  in  summer  in  contrast 
with  the  air  outside.  For  the  same  reason  water  of  a  tem- 
perature that  would  ordinarily  be  refreshingly  cool  to  us 
seems  warm  when  drunk  after  eating  ice-cream.  It  is  man- 
ifest, then,  that  there  is  no  fixed  dividing-line  between  heat 
and  cold ;  they  are  merely  relative  terms.  There  is,  in  fact, 
no  such  thing  as  cold.  Substances  are  cold  from  being 
deprived  of  heat ;  and  no  substance  ever  has  all  its  heat 
taken  from  it.  Sir  Humphry  Davy  proved  that  there  is 
heat  in  ice  by  rubbing  two  pieces  together  in  a  very  cold 
room  until  they  were  gradually  melted.  Now,  this  was  not 
done  by  the  air,  for  that  was  at  a  temperature  below  the 
freezing-point ;  the  heat  which  melted  the  ice  resided  in 
the  ice  itself. 


HEAT.  253 

165.  Nature  of  Heat. — We  have  just  stated,  that  there  is 
no  such  thing  as  cold,  and  we  now  assure  you  that  there  is 
no  such  thing  as  heat.  That  is  to  say,  there  is  no  substance 
to  that  which  we  call  heat.  A  hot  body  weighs  no  more 
than  the  same  body  after  it  has  cooled.  When  we  heat  a 
substance  we  add  nothing  to  it,  and  when  we  cool  it  we 
take  nothing  ponderable  from  it.  And  yet  the  effects  of 
heat  are  everywhere  present:  "  we  know  hot  iron,  hot  wa- 
ter, or  hot  air ;  but  nature  nowhere  presents  to  us,  nor 
has  art  succeeded  in  exhibiting  to  us,  heat  alone."  The 
old  theory  of  the  nature  of  heat  was  that  it  is  an  impon- 
derable, or  unweighable,  and  consequently  very  subtile 
substance,  pervading  all  matter,  and  tending  to  diffuse  by 
the  mutual  repulsion  of  the  particles.  This  view  has  given 
way  to  another  supposition,  now  generally  received,  that 
heat  is  merely  motion  of  a  certain  kind  among  the  material 
particles  of  bodies.  As  with  sound  (§  147),  this  motion  is  vi- 
bratory, and  the  width  and  velocity  of  the  vibrations  deter- 
mine the  temperature  of  the  body,  the  hottest  substances 
being  those  in  which  the  particles  vibrate  with  greatest 
rapidity.  It  is  further  assumed  that  the  transfer  oflie.it 
from  one  body  to  another  is  effected  by  means  of  an  im- 
ponderable elastic  and  subtile  fluid  called  ether,  which  fills 
all  space,  both  celestial  and  intermolecular  ( §  7 ).  This 
ether  transmits  with  immense  velocity  the  vibratory  mo- 
tion of  the  particles ;  and  it's  motion  produces  heat,  just  as 
the  motion  of  aeriform  bodies  produces  sound. 

According  to  the  old  view,  then,  heat  existed  as  a  kind 
of  matter,  called  "  caloric."  Under  the  new  view,  heat  is  a 
"  mode  of  motion." 

Many  philosophers  have  contributed  to  the  establishment  of  the  latter 
theory,  but  the  first  reliable  experiments  in  this  connection  were  made  in 
1798  by  our  countryman  Benjamin  Thompson,  better  known  as  Count 
liumford.  Having  entered  the  service  of  the  Elector  of  Bavaria,  he  had, 

L2 


254  NATURAL  PHILOSOPHY. 

among  other  duties,  charge  of  the  Munich  arsenal.  When  engaged  in  bor- 
ing cannon,  he  observed  the  enormous  amount  of  heat  generated,  a  phe- 
nomenon which  in  his  opinion  was  insufficiently  explained  by  the  common 
theory  that  threat  was  furnished  by  the  abrasion  of  the  metal.  He  ac- 
cordingly made  a  series  of  experiments  which  showed  that  enough  heat 
was  generated  in  boring  a  metallic  cylinder  (by  means  of  horse-power)  to 
raise  water  surrounding  it  to  boiling.  Reasoning  upon  these  remarkable 
results,  he  was  led  to  the  conclusion  that  the  heat  generated  could  not  be  a 
substance  or  material,  but  was  in  all  probability  motion.  Subsequently 
this  theory  has  received  confirmation  through  the  labors  of  many  distin- 
guished men,  and  it  has  been  further  proved  that  an  exact  relation  exists 
between  the  amount  of  heat  generated  and  the  amount  of  mechanical  force 
exerted  in  its  production.  Not  only,  however,  is  mechanical  force  capable 
of  being  transformed  into  heat,  but  the  latter  can  be  converted  into  the 
former ;  in  other  words,  they  are  mutually  convertible.  Whenever  motion 
is  arrested,  some  of  the  force  is  transformed  into  heat.  Of  this  we  have 
innumerable  examples :  a  blacksmith  hammers  a  piece  of  cold  iron  until  it 
glows  with  a  red  heat ;  a  bullet  fired  against  an  iron  target  flattens  out  and 
becomes  quite  warm. 

We  have  already  stated  that  force,  like  matter,  is  indestructible  (§  9). 
When  it  appears  to  be  destroyed,  as  in  the  case  of  arrested  motion,  it  is 
really  converted  into  some  other  manifestation  of  force.  Such  are  a  few 
of  the  phenomena  and  reasonings  which  have  aided  in  establishing  the 
present  theory  of  the  nature  of  heat.  You  will  learn  in  another  chapter 
that  heat  and  light,  as  well  as  electricity,  are  also  mutually  convertible, 
phenomena  on  which  is  based  the  grand  law  of  the  "  correlation  and  con- 
servation offeree." 

166.  Sources  of  Heat. — The  principal  source  of  heat  on 
our  earth  is  the  sun,  though  that  body  is  ninety -two 
millions  of  miles  distant  from  us.  As  the  heat,  in  travel- 
ling all  this  long  journey,  is  becoming  more  and  more  dif- 
fused or  scattered,  we  can  have  no  conception  of  the  in- 
tensity of  the  heat  in  the  sun  itself.  We  can,  however, 
form  an  approximate  idea  by  observing  the  effects  of  heat 
when  some  of  its  separated  rays  are  gathered  to  a  point 
by  a  powerful  lens,  as  represented  in  Fig.  221.  A  lens 
which  concentrated  the  heat  ten  thousand  times  melted 


HEAT.  255 

platinum,  gold,  quartz,  etc.,  in 
a  few  seconds.  And  since  the 
heat  at  the  sun  is  supposed  to 
be  vastly  more  intense  than 
this,  none  of  the  most  solid 
substances  of  our  earth  would 
remain  solid  if  present,  but 
many  of  them  would  become 
liquid,  and  others  even  vapors. 
The  heat  which  the  sun  con- 
stantly radiates  to  the  earth 

pervades,  all  substances,  producing  motion,  and  awakening 
life  everywhere;  so  that,  in  the  expressive  language  of  the 
Bible,  "There  is  nothing  hid  from  the  heat  thereof." 

Another  source  of  heat  is  within  the  earth  itself.  It  has 
been  found  that  as  we  descend  into  the  earth  the  tem- 
perature constantly  increases  the  farther  we  go.  This 
internal  heat  is  attributed  in  part  to  subterranean  fires 
and  various  chemical  actions.  Here  and  there  we  see 
external  evidences  of  this  in  the  eruptions  of  volcanoes, 
the  boiling  springs,  the  jets  of  steam  and  sulphurous 
vapors,  etc.  In  very  deep  mines  the  temperature  rises 
as  you  descend,  becoming  positively  uncomfortable  be- 
low a  depth  of  1800  to  2000  feet.  But  that  the  heat  in 
our  earth  which  comes  from  these  subterranean  sources  is 
small  compared  with  that  which  comes"  from  the  sun  is 
evident  from  the  fact  that  the  rate  of  increase  of  heat  at 
great  depths  is  much  less  than  it  is  nearer  the  surface. 
This  would  seem  to  show  that  although  fires  within  the 
earth  may  have  considerable  influence  in  heating  its  crust, 
on  which  we  live,  it  derives  the  most  of  its  heat  from  the 
sun,  at  least  to  a  very  great  depth. 

Another  very  common  source  of  heat  is  chemical  action. 
We  see  it  continually  produced  in  chemical  experiments. 


250 


NATURAL   PHILOSOPHY. 


Combustion,  which  is  the  development  of  heat  and  light 
accompanying  chemical  combination,  as  will  be  shown  to 
you  in  the  Second  Part  of  this  series,  is  the  most  common 
of  all  the  chemical  sources  of  heat.  Animal  heat  is  also,  for 
the  most  part,  a  result  of  chemical  action. 

Mechanical  action  is  a  common  source  of  heat.  The  rub- 
bing of  a  match  producing  heat  enough  to  occasion  flame 
is  a  familiar  example.  The  spark  produced  by  striking  to- 
gether flint  and  steel  is  an  incandescent  particle  of  steel  ig- 
nited by  the  blow.  The  American  Indians  were  accustomed 
to  procure  fire  by  rubbing  together  two  dry  sticks  until  they 
learned  an  easier  way  from  civilized  neighbors.  An  im- 
proved method  of  obtaining  fire  by  the  friction  of  wood 
against  wood  is  shown  in  Fig.  222.  The  board  B  is  pressed 


Fig.  222. 

strongly  against  A,  while  the  upright  piece  of  hard  wood 
is  rapidly  revolved  by  means  of  the  instrument  known  as 
a  "  fiddle-bow."  As  soon  as  there  are  any  indications  of 


HEAT.  257 

fire,  a  second  person  approaches  a  piece  of  tinder.  This 
affords  a  striking  example  of  the  conversion  of  motion  into 
heat  (§  165). 

The  blacksmith,  previous  to  the  invention  of  phosphorus 
matches,  often  lighted  his  fire  by  touching  a  sulphur  match 
to  a  nail  made  red-hot  by  rapid  and  continued  hammering. 
Machinery  has  sometimes  been  set  on  fire  by  friction,  and 
the  water  around  a  mass  of  metal  has  been  so  heated  by 
boring  as  even  to  boil  (§  165).  If  you  stretch  a  piece  of 
India-rubber  several  times  in  quick  succession,  and  then 
apply  it  to  your  lips,  you  will  perceive  that  the  motion  has 
warmed  it. 

Heat  is  sometimes  accompanied  by  light,  and  sometimes 
the  latter  force  is  absent :  its  presence  depends  upon  the 
rapidity  of  the  vibrations  communicated  to  the  ether  sur- 
rounding the  source  of  heat,  heat  waves  being  less  rapid 
than  those  of  light.  To  this  we  shall  again  refer  in  the 
chapter  on  Light  (§  211). 

167.  Expansion  of  Solids. — The  principal  effects  of  heat 
are  expansion,  liquefaction,  and  vaporization  ;  each  of  these 
requires  your  attention.  You  have  already  learned  in  §  5 
that  heat  acts  in  opposition  to  the  at- 
traction of  cohesion,  tending  to  sepa- 
rate the  particles,  and  so  produces  an 
expansion  of  any  substance.  This  may 
be  exemplified  in  the  experiment  repre- 
sented in  Fig.  223,  in  which  A  B  is  an 
iron  rod  of  such  a  size  that  at  the  or- 
dinary temperature  it  will  fit  into  the 
space  C  D  in  a  bar  of  iron,  and  easily 
pass  through  the  hole,  E.  If  the  rod  be 
heated,  it  will  be  enlarged  or  expanded 
in  all  directions,  so  that  it  will  neither 
fit  into  C  D  nor  pass  into  the  hole,  E.  Fig.  223. 


258  NATURAL   PHILOSOPHY. 

When  the  wheelwright  puts  a  tire  upon  a  wheel,  he  uses 
the  expansive  force  of  heat  to  make  it  fit  tightly  and 
firmly.  The  tire  is  purposely  made  a  little  too  small 
to  fit  the  wheel;  but  by  being  heated  it  is  so  expand- 
ed that  it  will  readily  go  around  the  wheel,  and  then 
in  contracting  as  it  cools  it  compresses  the  fellies  very 
tightly.  Water  is  poured  on  to  cool  the  iron  quickly,  and 
thus  prevent  it  from  burning  the  wood.  Iron  hoops  are 
put  on  barrels  in  a  similar  manner,  the  compression  caused 
by  their  contraction  binding  the  staves  together  very 
strongly.  In  like  manner  the  plates  of  boilers  are  fastened 
together;  the  rivets  are  put  in  red-hot,  so  that  by  their  con- 
traction they  may  bind  the  plates  closely  together.  If  an 
iron  gate  just  shuts  into  its  place  in  cold  weather,  expan- 
sion will  prevent  it  shutting  in  warm  weather.  In  order 
to  avoid  this  difficulty,  allowance  must  be  made  in  fitting 
it  for  the  expansion  to  which  it  will  be  subjected  by  heat. 
So  in  laying  the  rails  of  a  railroad  in  cold  weather  care 
must  be  taken  not  to  put  the  ends  too  near  together. 
Nails  often  become  loose  after  the  lapse  of  years  from  the 
wear  of  the  wood  around  them,  occasioned  by  their  alter- 
nate expansion  and  contraction.  The  leaking  of  gas-pipes 
in  the  earth  is  often  undoubtedly  caused  by  the  loosening 
of  the  joints  from  contraction  and  expansion  of  the  pipes 
by  varying  temperatures  of  the  soil,  especially  when  not 
laid  very  deep.  If  a  stopper  stick  fast  in  a  bottle,  it  can 
sometimes  be  loosened  by  applying  to  the  neck  a  cloth 
dipped  in  hot  water,  because  the  neck  becomes  expanded 
at  once  by  the  heat. 

A  similar  expedient  was  once  very  ingeniously  made  use  of  in  repairing 
the  machinery  of  the  steamer  Persia  at  sea,  and  was  perhaps  the  means  of 
saving  the  vessel  and  the  lives  of  all  on  board.  The  accident  which  oc- 
curred was  the  breaking  of  the  port  crank-pin  of  the  engine.  The  prob- 
lem to  be  solved  was  the  removal  of  this  pin,  which  weighed  nearly  a  ton, 


HEAT. 


259 


and  the  substitution  of  a  sound  one  which  they  had  on  hand  in  its  place. 
But  it  was  found  impossible  to  start  the  broken  pin  from  its  socket  with 
all  the  force  which  could  be  brought  to  bear  upon  it  by  a  sort  of  battering- 
ram  constructed  extemporaneously  for  the  purpose.  It  was  then  deter- 
mined to  try  the  expansive  force  of  heat.  An  iron  platform  was  built 
under  the  socket,  and  a  hot  fire  made  upon  it.  The  socket  soon  expand- 
ed, and  the  pin  was  then  readily  knocked  out  by  the  battering-ram,  just  as 
the  stopper  of  a  bottle  is  easily  removed  when  the  neck  is  heated. 

The  walls  of  a  very  large  building  in  Paris,  which  had 
bulged  out  and  were  in  danger  of  falling,  were  restored  to 
their  upright  position  by  the  expansion  of  iron.  It  was 
done  in  this  way: 
Long  rods  of  iron 
were  run  through 
the  walls  after  the 
plan  represented  in 
Fig.  224,  their  ends 
being  made  with  a 
screw-thread,  with 
nuts  fitted  to  them. 
Alternate  rods  were 
first  heated,  and  as 
they  lengthened  the 
nuts  were  screwed 
up  tight  to  the 
walls.  On  cooling,  Fig.  224. 

their  contraction  would  of  course  draw -the  walls  together. 
The  other  bars  were  then  heated  and  managed  in  the  same 
way.  The  one  set,  you  see,  were  made  to  hold  on  by  their 
nuts  to  what  had  already  been  gained,  while  the  other  were 
expanding.  By  many  repetitions  of  this  process  the  walls 
were  straightened  and  the  building  saved.  The  same  mode 
has  been  adopted  successfully  in  other  cases  of  a  similar 
character. 

Different  substances  expand  at  different  rates ;  copper 


260  NATURAL   PHILOSOPHY. 

expands  more  than  twice  as  much  as  glass,  and  zinc  nearly 
twice  as  much  as  copper,  with  the  same  increase  of  temper- 
ature. Advantage  is  taken  of  the  unequal  expansion  of 
metals  to  regulate  the  length  of  pendulums,  upon  which, 
as  you  learned  in  §  93,  depends  the  correctness  of  time- 
keepers. Various  arrangements  are  adopted,  but  that 
e  known  as  the  gridiron  pendulum  is  the  best,  in 
which  ingenious  use  is  made  of  the  fact  that  heat 
/«•  expands  brass  nearly  twice  as  much  as  it  does  steel. 
A  simple  form  of  this  pendulum  is  given  in  Fig.  225. 
The  middle  rod  is  made  of  brass,  and  the  side  rods,  b 
and  c,  of  steel.  Suppose  that  the  brass  rod  expands 
or  increases  in  length  half  an  inch.  The  rod  c  would 
be  drawn  upward  by  it,  and  the  rod  b  down- 
ward, each  one  quarter  of  an  inch ;  but  this 
effect  is  counteracted  by  the  expansion  of 
Fig.  225.  each  steel  rod,  which  is  half  that  of  the  brass 
— that  is,  one  quarter  of  an  inch.  The  ball,  c?,  there- 
fore, always  retains  the  same  distance  from  the 
point  of  suspension,  e.  Fig.  226  represents  a  grid- 
iron pendulum  of  a  more  complex  character,  part 
of  the  bars  being  steel  and  a  part  brass. 

168.  Expansion  of  Liquids. — Liquids  are  expand- 
ed by  heat  to  a  greater  degree  than  solids,  but 
very  unequally  so.  Thus  water  is  expanded 
more  than  twice  as  much  as  mercury,  and  al- 
cohol six  times  as  much.  We  have  a  frequent 
example  of  the  expansion  of  water  by  heat  in  Fis-226- 
our  kitchens.  If  the  tea-kettle  be  put  over  the  fire  filled 
to  the  brim,  it  will  run  over  long  before  the  water  be- 
gins to  boil.  All  liquids  occupy  more  space  in  summer 
than  in  winter,  and  in  the  former  case  weigh  less — that 
is,  have  less  of  real  substance  in  them  than  in  the  latter. 
If,  therefore,  alcohol,  or  oil,  or  molasses  be  bought  by  the 


HEAT. 


2C1 


Fig.  227. 


gallon  in  winter  and  sold  in  sum- 
mer, there  will  be  a  profit  afforded 
by  the  expansion. 

The  influence  of  the  expansion  of  heat  upon 
the  specific  gravity  of  liquids  may  be  very 
prettily  shown  by  the  following  experiment: 
Throw  some  little  bits  of  amber  —  a  sub- 
stance having  nearly  the  same  specific  grav- 
ity as  water  —  into  water  in  a  glass  ves- 
sel, and  heat  the  water  by  a  spirit-lamp, 
as  represented  in  Fig.  227.  That  portion 
of  the  water  which  is  heated  passes  up- 
ward because  it  becomes  specifically  lighter, 
and  colder  water  continually  comes  down  to 
take  its  place.  An  upward  current  passes 
up  in  the  middle,  as  indicated  in  the  picture, 
the  downward  coming  down  at  the  sides. 
This  will  be  made  manifest  by  the  little 
bits  of  amber.  This  experiment  also  illus- 
trates the  manner  in  which  heat  is  transmitted  in  liquids  by  convection,  as 
will  be  more  fully  explained  in  §  183,  Chapter  XVI. 

169.  Thermometers. — The  expansion  of  liquids  by  heat 
affords  us  a  convenient  means  of  measuring  differences  of 
temperature:  the  instrument  commonly  employed  is  called 
ia  thermometer,  the  word  being  derived  from  two  Greek 
words  signifying  together  "  heat  -  measurer."  The  form 
and  general  construction  of  a  thermometer  are  familiar  to 
all ;  the  liquids  used  for  filling  the  bulbv  are  alcohol  and 
mercury.  The  latter  answers  well  except  in  the  extreme 
cold  of  the  polar  regions ;  for  mercury  becomes  solid  at 
about  39  degrees  below  zero,  while  alcohol  cannot  be  fro- 
zen by  any  known  degree  of  cold.  The  manner  in  which 
a  thermometer  indicates  temperatures  is  very  simple :  heat 
expands  the  liquid  in  the  bulb,  and  the  only  way  in  which 
it  can  occupy  more  space  is  by  rising  in  the  tube.  The 
removal  of  heat,  on  the  other  hand,  causes  contraction 


2G2  NATURAL   PHILOSOPHY. 

and  of  course  a  proportionate  fall  of  the  mercurial 
column. 

|10Q.  170.  Fahrenheit's  Thermometer. — The  thermome- 
ter was  invented  in  the  beginning  of  the  seven- 
teenth century,  but  it  is  not  decided  who  was  the 
inventor.  There  may  have  been  in  this  case,  as  in 
others,  more  inventors  than  one,  the  same  ideas  hav- 
ing, perhaps,  entered  several  inquiring  minds  at  the 
same  time.  Various  fluids  were  used  by  different 
persons.  Sir  Isaac  Newton  used  linseed-oil.  Fahr- 
enheit, a  native  of  Hamburg,  who  flourished  in  the 
first  part  of  the  last  century,  was  the  first  to  use 
mercury.  Though  various  propositions  were  made 
by  Newton  and  others  in  regard  to  the  measure- 
ment of  heat  by  thermometers,  no  thermometric 
|°  scale  seems  to  have  met  with  general  reception 
till  that  of  Fahrenheit,  which  was  introduced  about 
1720.  His  zero  is  the  point  at  which  the  mercury 
stood  in  the  coldest  freezing  mixture  that  he  could 
make ;  and  he  supposed  that  this  was  the  greatest 
possible  degree  of  cold,  as  it  was  the  greatest  that 
he  knew.  He  next  found  the  point  at  which  the 
Fig.  223.  mercury  stood  in  melting  ice.  This  he  called  the 
freezing-point,  because  the  temperature  is  the  same  in 
water  passing  into  the  solid  from  the  fluid  state  as 
in  Avater  passing  into  the  fluid  state  from  the  solid.  In 
other  words,  this  point  in  the  scale  marks  the  transition 
line  between  the  two  states.  From  this  point  Fahrenheit 
marked  off  32  equal  spaces  or  degrees  down  to  zero.  He 
then  found  the  point  at  which  the  mercury  stands  in  boil- 
ing water,  and  called  this  the  boiling  -  point.  Marking 
off  the  space  on  the  scale  between  this  and  the  freezing- 
point  in  the  same  manner,  there  are  180  degrees — that  is, 
the  boiling-point  is  212  degrees  above  zero.  The  degrees 


HEAT. 


2G3 


above  zero  are  commonly  designated 
by  the  mark  +,  plus;  and  those  below 
by  the  mark  — ,  minus.  Thus,  +32° 
signifies  32  degrees  above  zero,  and 
—  32°  signifies  32  degrees  below. 

171.  Thermometric  Scales. — Fahr- 
enheit's thermometer  is  the  one  com- 
monly used  in  this  country.  But  there 
are  several  other  thermometers  on  dif- 
ferent scales,  as  the  Centigrade,  Reau- 
mur's, and  De  Lisle's.  Fig.  229  shows 
the  scales  of  these  thermometers 
placed  side  by  side.  In  the  Centi- 
grade thermometer,  which  is  in  use  in 
France,  and  indeed  in  a  large  part  of 
Europe,  the  zero  is  placed  at  the  freez- 
ing-point ;  and  the  space  between  this 
and  the  boiling-point  is  divided  into 
100  degrees,  which  gives  it  the  name 
Centigrade.  It  is  also  called  Celsius's, 
after  its  inventor.  Reaumur's,  which 
is  in  use  in  Germany,  has  the  same 
zero-point,  but  has  only  80  degrees 
from  this  to  the  boiling-point.  De 
Lisle's  which  has  gone  entirely  out  of 
use,  has  its  zero  at  the  boiling-point. 
In  the  arrangement  of  Fahrenheit  the 
zero  is  a  mere  arbitrary  point,  and 
the  division  of  the  scale  into  212  parts 
is  very  inconvenient.  The  Centigrade 
thermometer,  on  the  other  hand,  hav- 
ing two  points  easily  determined  and 
invariable — viz.,  that  of  the  freezing 
and  boiling  of  water — having  also  a 


Fig.  229. 


264 


NATUKAL   PHILOSOPHY. 


centesimal  scale,  possesses  great  advantages  for  scientific 
and  exact  investigations  over  every  other  style.  It  is  now 
used  by  scientific  men  almost  exclusively,  and  is  the  stand- 
ard adopted  in  this  series  of  works. 

The  following  short  table  may  be  useful  for  comparing  temperatures 
given  in  Fahrenheit  and  Centigrade  degrees : 


c. 

F. 

C. 

F. 

C. 

F. 

C. 

F. 

-20° 

-4° 

15° 

59° 

45° 

113° 

75° 

167° 

-15 

+5 

20 

68 

50 

122 

80 

176 

-10 

+  14 

25 

77 

55 

131 

85 

185 

-5 

+  23 

30 

86 

60 

140 

90 

194 

0 

32 

35 

95 

65 

149 

95 

203 

+  5 

41 

40 

104 

70 

158 

100 

212 

10 

50 

For  temperatures  not  given  in  the  above  table  the  following  rules  may 
be  used:  to  convert  degrees  on  Fahrenheit's  scale  to  corresponding  de- 
grees on  the  Centigrade  scale  subtract  32°,  multiply  the  remainder  by  5, 
and  divide  the  product  by  9 ;  to  convert  Centigrade  to  Fahrenheit  mul- 
tiply by  9,  divide  the  product  by  5,  and  add  32. 

172.  Expansion  in  Aeriform  Substances. — Heat  produces 
a  vastly  greater  expansive  effect  in  air,  the  gases,  and  va- 
pors than  it  does  in  liquids. 
The  expansion  of  air  by  heat 
may  be  shown  very  prettily  in 
this  way:  Take  a  glass  tube 
having  a  bulb  on  one  end, 
and,  placing  the  other  open 
end  in  water  (as  represented 
in  Fig.  230),  apply  the  palm 
of  your  hand  to  the  bulb. 
The  heat  of  the  hand,  being 
communicated  to  the  bulb, 
will  expand  the  air,  and 
bubbles  of  air  will  escape 
through  the  water.  On  re- 


HEAT.  265 

moving  the  hand,  and  allowing  the  bulb  to  cool,  the  air 
in  it  will  be  condensed,  and  water  will  enter  the  tube 
in  proportion  to  the  amount  of  air  which  has  escaped.  A 
bladder  partly  filled  with  air  will  swell  out  to  plumpness 
if  heated  sufficiently,  and  a  full  one  may  be  so  heated  as 
to  burst  from  the  expansion  of  the  air.  Chestnut  and 
other  porous  wood  snap  very  much  when  burned,  be- 
cause the  heat  expands  the  air  and  moisture  contained 
in  their  pores. 

Balloons. — Tho  first  balloons  used  were  filled  with  heated  air.  You 
have  already  seen,  in  §  129,  why  balloons  rise.  The  hot-air  balloon  be- 
comes lighter  than  the  surrounding  atmosphere,  because  the  contained  air 
is  expanded  by  heat.  Of  course  such  a  balloon  is  not  so  effective  as  the 
gas  balloon,  for  the  air  within  it  loses  its  comparative  lightness  as  it  be- 
comes cooled ;  while  the  coal  gas  used,  being  very  much  lighter  than  air  at 
the  same  temperature,  does  not  lose  its  lightness  as  the  balloon  ascends. 
You  learned  in  §  131  that  the  atmosphere  becomes  thinner  as  we  go  up- 
ward. The  gas  balloon,  therefore,  rises  until  it  arrives  at  that  point  where 
the  air  is  of  about  the  same  specific  gravity  with  the  gas,  and  there  it  stops. 
It  is  made  to  descend  by  letting  out  some  of  the  gas  from  a  valve.  Gas 
was  not  used  for  balloons  till  1782.  Hydrogen  gas  was  employed  at  first, 
being  over  fourteen  times  lighter  than  air.  Of  late  the  common  coal-gas, 
carburetted  hydrogen,  is  generally  used,  because  it  can  be  so  readily  ob- 
tained from  gas-works. 

173.  Currents  in  the  Air  from  Heat. — Heat  is  the  grand 
mover  of  the  atmosphere.  Any  portion  of  it  that  be- 
comes warmer  than  surrounding  portion^  rises,  or  rather 
is  pushed  up,  for  the  same  reason  that  a  hot-air  balloon 
rises,  the  only  difference  between  the  two  cases  being 
that  in  the  one  the  air  is  confined,  and  in  the  other  is  left 
free,  and  so  becomes  diffused.  And  it  is  this  expansion 
that  causes  nearly  all  the  movements  witnessed  in  the 
air.  We  see  this  exemplified  in  various  ways  wherever 
there  is  a  fire.  The  air  heated  by  the  fire  is  forced  upward 
by  the  colder  air,  which,  on  the  principle  of  specific  gravity, 


266  NATUIIAL   PHILOSOPHY. 

seeks  to  get  below  the  warmer  and  lighter  air.  The  hot 
air  that  comes  through  the  registers  of  a  furnace  is  pushed 
up  by  colder  air  below.  For  the  same  reason  the  heated 
air  around  a  stove-pipe  is  constantly  ris- 
ing. This  is  very  prettily  shown  by  the 
toy  represented  in  Fig.  231,  which  is  a 
paper  cut  spirally,  and  suspended  upon 
the  point  of  a  wire.  The  upward  current 
makes  the  paper  revolve  rapidly  around 
the  wire.  It  is  owing  to  the  rising  of  heated 
air  that  the  galleries  of  a  church  are  warm- 
er than  the  space  below.  In  a  common 
room  the  air  is  so  disposed  that  its  warm- 
est portions  are  above  and  the  colder  below.  For  this 
reason  our  arrangements  for  producing  or  introducing  heat 
are  placed  at  as  low  a  point  as  possible. 

Chimneys. — We  speak  of  the  drautjJit  of  a  chimney,  and  we  say  of  one 
which  does  not  smoke  that  it  draws  well,  as  if  the  smoke  were  in  some 
way  actually  drawn  np.  But  the  same  principles  apply  here  as  those  above 
developed.  The  smoke,  which  is  a  combination  of  heated  air  and  gases, 
with  some  solid  matters  in  a  fine  state,  is  forced  up  the  chimney.  When 
a  chimney  does  not  draw  well,  we  open  a  door  or  a  window  for  a  little 
while  until  the  fire  is  well  started.  This  is  in  order  that  we  may  let  denser 
air  into  the  room,  so  that  the  smoke  may  be  pushed  up  more  forcibly. 
When  the  chimney  becomes  well  heated  there  is  ordinarily  no  difficulty, 
because  then  the  smoke  in  it  is  not  obliged  to  part  with  much  of  its  heat 
to  the  walls  of  the  chimney,  and  therefore  is  so  much  lighter  than  the  air 
in  the  room  that  it  is  very  easily  forced  upward.  The  principal  reason 
that  a  stove-pipe  generally  draws  better  than  a  chimney  is  that  there  is 
much  less  heat  expended  in  establishing  and  maintaining  the  upward  cur- 
rent. Especially  is  this  true  if  the  chimney  be  a  large  one.  In  such  a 
case  both  a  great  extent  of  brick  and  a  large  body  of  air  must  be  heated 
to  establish  an  upward  current.* 

*  The  author  was  once  consulted  in  regard  to  a  smoking  stove.  It  was 
an  open  Franklin  stove,  the'  pipe  of  which  went  through  a  fire-board  into 


HEAT. 


267 


Fig. 232. 


174.  Winds.  —  If  you  open  the  door  of  a  heated  room, 
the  flame  of  a  caudle  held  near  the  floor  will  be  blown 
inward,  while  one  held  near 
the  top  of  the  door  will  have 
its  flame  blown  towards  the 
cold  entry.  (Fig.  232.)  This  is 
a  good  illustration  of  the  man- 
ner in  which  winds  are  pro- 
duced. Wherever  the  wind 
blows  it  is  caused  by  air  push- 
ing out  of  the  way  other  air 
that  is  warmer,  in  order  that  it 
may,  in  obedience  to  gravitation, 
get  as  near  the  earth  as  possible. 
Take,  for  example,  the  land  and 
sea  breezes,  as  they  are  called. 
During  a  hot  summer's  day  the  sun  heats  the  earth  power- 
fully, while  the  ocean  receives  but  little  of  its  heat.  The 
heated  land  heats  the  air  above  it ;  and  as  the  air  over  the 
ocean  is  cooler,  and  therefore  heavier,  it  pushes  upward  the 
air  of  the  land,  for  the  same  reason  that  water  pushes  up 
oil ;  and  as  this  goes  on  continuously,  a  regular  current  is  es- 
tablished. The  wind  blows  in  upon  the  land,  as  represented 
in  Fig.  233,  while  the  warmer  air  passes  upward  into  the 
higher  regions  of  the  atmosphere,  and  turns  towards  the 
sea.  The  arrows  show  the  course  of  the  currents.  The  re- 
semblance of  all  this  to  the  effect  upon  the  candle  held 
near  the  open  door  is  very  obvious,  the  cold  air  from  with- 
out blowing  in  below  representing  the  breeze  from  the 
ocean,  and  the  warm  air  of  the  room  blowing  out  above 

an  enormous  chimney.  He  recommended  that  a  pipe  with  a  knee  should 
extend  from  the  pipe  of  the  stove  a  little  way  up  the  chimney.  The  expe- 
dient was  successful,  because  but  a  small  body  of  air,  that  in  the  pipe, 
needed  to  be  heated  to  establish  an  upward  current. 


268 


NATURAL   PHILOSOPHY. 


DURING  DAY. 


t  t 
tt 
t  t 
t  t 


Fig.  233. 

representing  the  passage  of  the  warm  air  of  the  land  out 
towards  the  ocean.  At  night  this  is  apt  to  be  reversed. 
The  earth  becomes  cooled,  and  with  it  the  air  above.  The 
result  is  that  the  cooled  air  of  the  land  then  pushes  upward 
the  warmer  air  of  the  sea,  as  shown  in  Fig.  234. 


Fi".  234. 


HEAT. 


269 


1 75.  Winds  Affected  by  the  Rotation  of  the  Earth. — The  heat 
of  the  vertical  sun  in  the  tropics  causes  a  rise  of  heated  air  into  the  upper 
regions,  while  there  is  a  rush  of  colder  air  towards  the  equator  from  both 
north  and  south.  This  effect  is  represented  in  Fig.  235,  E  being  the  sun, 


Fig.  235. 

N  the  north  pole,  and  S  the  south  pole.  An  effect  similar  to  that  repre- 
sented in  Figs.  233  and  234  is  produced  here,  but  it  is  on  a  much  larger 
sciile.  But  the  diagram  does  not  present  the  matter  in  its  true  light  in  all 
respects.  The  prevailing  winds  in  the  equatorial  regions  are  not  north 
and  south  winds,  as  would  appear  from  this  diagram ;  but  they  are  from 
the  northeast  and  southeast.  Fig. 
236  will  explain  this.  As  the  earth 
turns  on  its  axis,  it  is  plain  that  there 
is  no  part  of  the  surface  of  the  earth 
that  moves  so  rapidly  as  the  equator, 
E  W,  for  that  moves  in  the  largest 
circle.  And  the  nearer  you  go  to 
either  pole,  N  or  S,  the  less  is  the 
rapidity  of  the  revolution.  Now,  the 
atmosphere  partakes  of  the  motion 
of  the  earth ;  the  air,  therefore,  at 
the  equator  is  moving  from  west  to 
east  with  the  earth  faster  than  any-  FJO-.  236. 

M 


270  NATURAL   PHILOSOPHY. 

where  else,  and  the  nearer  you  go  to  either  pole,  the  slower  its  motion. 
Hence  any  portion  of  air  blowing  from  the  north  or  the  south  towards 
the  equator,  and  coming  from  a  point  where  it  was  moving  east  slower 
than  air  at  the  equator,  would  from  its  lesser  momentum  lag  behind  the 
air  of  the  equator ;  and  the  wind  would  be  curved  towards  the  west,  as 
indicated  by  the  arrows.  The  result  would  be  that  the  northern  wind 
would  be  converted  into  a  northeaster,  and  the  southern  into  a  south- 
easter. All  this  can  be  made  more  clear  with  a  globe,  or,  indeed,  with 
any  round  object. 

176.  Liquefaction. — The  change  of  solids  into  liquids  is 
one  of  the  most  noticeable  effects  of  heat.     This  change 
requires  different  degrees  of  heat  in  different  substances. 
Thus  while  iron  melts  at  the  high  heat  of  1530°,  lead  melts 
at  330°,  sulphur  at  115°,  ice  at  0°,  and  mercury  at  39.4°  be- 
low zero.     Mercury  is  never  found  in  a  solid  state,  but  it 
sometimes  becomes  solid  in  the  arctic  regions  when  carried 
there  and  exposed  in  the  open  air.    We  are  apt  to  think  of 
water  as  in  a  more  natural  state  when  liquid  than  when 
solid,  just  as  we  think  of  iron  as  naturally  solid  and  mer- 
cury liquid.     But  in  all  these  cases  the  state  of  the  sub- 
stance depends  on  its  temperature,  and  this  is  varied  by 
circumstances.    Water  at  the  equator  is  always  liquid,  and 
the  idea  of  ice  there  is  exceedingly  unnatural ;  while  near 
the  poles  it  is  the  reverse,  ice  and  snow  reigning  every- 
where throughout  the  whole  year. 

177.  Evaporation. — There   are  two  ways   in  which  the 
change  of  a  liquid  into  a  vapor  occurs.     One  is  a  rapid 
change  when  heat  is  so  applied  as  to  raise  the  liquid  to  its 
boiling-point.    This  is  commonly  termed  vaporization.    The 
other  mode  is  the  ordinary  gradual  evaporation  which  goes 
on  from  the  surface  of  the  liquid.    This  process  is  going  on 
continuously,  not  requiring  any  particular  degree  of  heat, 
but  occurring  under  all  degrees  of  the  temperature  of  a 
liquid.     Its  rapidity,  however,  is  in  proportion  to  the  de- 
gree of  heat,  as  may  be  seen  by  the  rise  of  vapor  from 


HEAT.  271 

heated  water  long  before  it  begins  to  boil.  The  same 
thing  can  also  be  seen  on  a  bright  summer's  morning, 
when  the  heat  of  the  sun  causes  the  moisture  gathered 
from  rain  or  dew  to  rise  so  abundantly  from  fences,  boards, 
and  roofs  as  to  be  visible  like  smoke. 

178.  Moisture  in  the  Atmosphere.  —  Evaporation  is  con- 
stantly going  on  from  every  wet  surface,  except  when  the 
air  is  so  loaded  with  moisture  that  it  can  take  up  no  more. 
The  vapor  is  not  ordinarily  visible,  the  particles  of  water 
passing  quietly  upward  among  those  of  the  air,  being  min- 
gled with  the  air  just  as  some  liquids  mix  with  water.  It 
becomes  visible  only  when  so  much  of  it  rises  that  the 
mixture  of  water  and  air  is  not  readily  effected.  The  read- 
iness with  which  this  takes  place  depends  much  upon  the 
temperature  of  the  atmosphere.  Some  very  common  phe- 
nomena illustrate  this.  In  a  very  cold  day  the  breath  of 
animals,  as  it  comes  out  of  the  mouth,  seems  to  be  load- 
ed with  moisture.  Why  ?  It  is  not  because  it  contains 
more  moisture  than  in  warm  weather,  but  because  cold  air 
condenses  the  aeriform  water  and  renders  it  visible  in  mi- 
nute drops.  The  same  explanation  applies  to  the  smoking 
of  wet  fences  and  roofs' in  the  sun  of  a  summer's  mornino-. 

^ 

The  moisture  is  heated  by  the  sun,  but  the  air,  not  having 
become  very  warm  as  yet,  cannot  readily  convert  into  va- 
por all  the  moisture  that  rises.  The  phenomenon  is  not 
apt  to  occur  when  the  hot  sun  shines  after  a  shower  at 
midday  or  in  the  afternoon,  because  then  the  air  is  warm 
enough  to  take  up  all  the  moisture  present. 
-  179.  Clouds. — The  water  which  rises  in  the  air  by  evap- 
oration is  variously  disposed  of.  Some  of  it  is  deposited 
as  dew  or  frost.  Some  of  it  forms  fog.  Some  of  it  also 
mounts  far  upward  and  forms  the  clouds,  which  are  real- 
ly collections  of  fog  high  up  in  the  air.  In  fog  and  in 
clouds  the  water  which  in  its  evaporation  is  invisible  be- 


272  NATURAL   PHILOSOPHY. 

comes  visible.  Let  us  see  how  this  is.  The  atmosphere  al- 
ways contains  more  or  less  water,  but  the  particles  are  so 
minutely  divided  and  so  thoroughly  mingled  with  the  parti- 
cles of  the  air  that  they  cannot  be  seen.  But  in  a  fog  or 
cloud  the  particles  of  water  are  gathered  together  in  little 
clusters,  as  we  may  express  it.  And  it  is  supposed,  some 
think  ascertained,  that  each  of  these  clusters  of  particles 
is  globular  and  hollow.  If  so,  then  we  may  regard  every 
cloud  as  a  vast  collection  of  minute  bubbles  or  balloons 
careering  through  the  air. 

/Shapes  of  Clouds. — Clouds  assume  a  very  great  variety  of 
shapes,  the  causes  of  which  arc  for  the  most  part  not  under- 
stood. They  are  generally  divided  into  four  classes :  cir- 
rus, cumulus,  stratus,  and  nimbus.  Besides  these  there  are 
several  intermediate  forms  known  as  cirro-cumulus,  cirro- 
stratus,  and  cumulo-  stratus.  All  these  forms  except  the 
nimbus  are  shown  in  the  full-page  engraving  Fig.  237;  the 
cirrus  being  marked  by  one  bird,  the  cirro-cumulus  by  two, 
the  cirro-stratus  by  three,  the  cumulus  by  four,  the  cumulo- 
stratus  by  five,  and  the  stratus  by  six. 

The  cirrus  is  a  light,  fleecy  cloud,  having  graceful  curves 
like  curls,  and  hence  its  name,  which  is  the  Latin  word  for 
curl.  Such  clouds  are  commonly  very  high  up  in  the  air. 
It  is  the  first  cloud  to  appear  after  a  period  of  fine  weather, 
its  delicate,  waving,  and  thread-like  forms  stretching  across 
the  blue  sky  like  pencilled  lines  of  white.  Although  this 
cloud  appears  so  light  and  airy,  it  is  probably  composed  of 
minute  masses  of  ice,  for  at  the  enormous  distance  at  which 
it  floats  along  the  earth  the  temperature  is  very  low  even 
in  summer. 

The  cumulus  appears  as  heaps  rounded  upward,  often 
looking  like  mountains  of  snow  when  they  are  illuminated 
by  the  sun.  The  name  is  derived  from  the  Latin  for  heap. 
In  Fio-.  237  this  form  is  marked  by  four  birds.  It  is  a  cloud 


HEAT. 


273 


Fis. 237. 


274 


NATURAL   PHILOSOPHY. 


of  dense  structure,  and  forms  and  floats  in  the  lower  re- 
gions of  the  atmosphere.  It  has  been  called  the  "cloud  of 
day,"  being  produced  in  the  daytime  by  the  currents  of 
moist  warm  air  rising  from  the  heated  earth. 

The  stratus,  from  the  Latin  for  a  layer,  is  marked  by  six 
birds.  Clouds  of  this  form  lie  low  in  the  horizon,  stretched 
along  like  a  sheet.  It  has  been  called  the  "  cloud  of  night," 
because  it  usually  forms  towards  nightfall,  grows  denser 
during  the  night,  and  is  dissipated  shortly  after  sunrise. 
It  is  caused  by  the  vapors  that  rise  during  the  day,  and 
descend  at  evening  with  the  falling  temperature. 

The  nimbus,  or  rain-cloud,  is  represented  in  Fig.  238 ;  it 


Pig. 233. 

has  a  uniform  gray  or  dark  color.  This  form  of  cloud  is 
also  called  the  cumulo-cirro-stratus,  a  name  suggesting  the 
way  in  which  it  is  formed,  being  a  combination  of  the  three 
named. 

The  remaining  complex  forms  we  need  only  briefly  notice :  the  cirro-cu- 
mulus (marked  by  two  birds,  Fi^.  237)  is  commonly  called  the  "  mackerel- 
sky,  "  and  is  regarded  as  a  quite  sure  indication  of  approaching  rain.  The  cu- 
mulo-stratiis  (five  birds)  is  formed  of  small  fleecy  clouds  surrounding  the  cu- 
mulus, and  often  precedes  a  storm  ;  this  form  is  sometimes  called  "thunder- 


HEAT.  275 

heads."    The  cirro-stratiis  (three  birds)  is  sufficiently  explained  by  the  en- 
graving. 

Water  is  gathered  into  clouds  undoubtedly,  in  part  at  least,  through 
the  influence  of  attraction.  But  what  circumstances  cause  these  vari- 
ous shapes  is  not  known.  Whatever  they  are,  their  influence  is  some- 
times very  extensive,  giving  a  similar  shape  to  all  the  clouds  covering  the 
whole  arch  of  the  heavens  ;  and  at  other  times  they  operate  variously 
in  different  localities,  producing  different  shapes,  sometimes  even  quite 
near  each  other.  Sometimes  the  edge  of  a  cloud  is  irregular,  or  curved, 
or  feathery ;  and  at  others  it  is  a  well-defined  line,  stretching  along  over 
a  large  portion  of  the  horizon.  In  all  these  cases  we  have  only  divers  ar- 
rangements of  the  same  thing — a  collection  of  vesicles  of  water  contain- 
ing air,  whicli  is  made  lighter  than  the  air  outside  of  the  cloud  by  means 
which  we  shall  explain  in  the  next  chapter. 

180.  Rain,  Snow,  and  Hail. — When  it  rains,  the  vesicles 
or  minute  bubbles  of  which  the  clouds  are  composed  are 
broken  up,  and  each  drop  of  rain  contains  the  water 
from  a  multitude  of  these  vesicles.  But  let  us  see  ex- 
actly how  this  result  is  produced.  Rain  results  from  the 
contraction  of  the  clouds  by  cold.  A  cold  current  of  air 
coming  in  contact  with  a  cloud  will  condense  its  bubbles 
into  drops,  and  these,  of  course,  will  fall.  The  same  result 
occurs  if  a  cloud  passes  into  a  cold  stratum  of  air.  The 
first  effect  of  cold,  upon  the  bubbles  may  be  made  clear  by 
Fig.  239.  If  a  bubble  be  contracted  by  the  influence  of 
cold,  the  water  of  its  wall  being  made  thicker, 
gravitation  will  cause  a  gathering  at  the,  lower 
part,  as  represented  by  the  dotted  line.  You 
often  see  a  similar  effect  in  the  soap-bubble ; 
it  rises  filled  with  warm  air  from  your  lungs, 
and  as  it  ascends  is  contracted  by  the  colder 
air  around  it.  This  contraction  makes  the  water  hang  from 
the  bottom.  And  as  the  soap-bubble  at  length  bursts  in  the 
air  from  the  weight  of  this  water,  so  it  is  with  the  vesicles 
in  the  cloud.  And  many  of  these,  united  together  by  attrac- 


270  NATURAL   PHILOSOPHY. 

lion,  form  a  drop.  When  the  cold  is  sufficiently  severe,  it 
makes  the  water  of  the  ruptured  vesicles  of  a  cloud  ar- 
range itself  in  snow-crystals  instead  of  drops.  And  when 
cold  acts  with  great  rapidity  upon  a  cloud,  it  presses  the 
particles  of  water  together  so  suddenly  that  there  is  no 
time  for  them  to  assume  a  crystalline  arrangement,  and 
hail  is  formed. 

181.  Vaporization. — The  production  of  vapor  by  boiling 
differs  in  some  respects  from  quiet  evaporation.  Here  the 
liquid  is  raised  in  temperature  to  its  boiling-point,  and  the 
formation  of  vapor  is  not  confined  to  the  surface.  The 
liquid  boils  when  the  small  bubbles  of  aeriform  matter, 
forming  at  the  bottom  of  the  vessel,  rise  to  the  surface  and 
thereby  keep  the  liquid  in  a  state  of  agitation.  The  tem- 
perature at  which  liquids  boil  varies  for  each ;  thus  the 
boiling-point  of  water  is  100°  Centigrade  (or  212°  Fahren- 
heit), that  of  alcohol  78.4°,  that  of  ether  35°,  that  of  oil 
of  turpentine  165°,  and  that  of  mercury  360°  Centigrade. 

The  temperature  at  which  a  given  liquid  boils  depends 
upon  the  amount  of  heat  which  it  requires  to  overcome 
both  the  natural  attraction  of  its  particles  and  the  com- 
pressing force  of  the  atmosphere.  Pressure  restrains  the 
production  of  vapor,  whether  it  be  formed  by  evaporation 
or  vaporization.  We  know  by  experiments  with  the  air- 
pump  that  the  less  the  pressure  of  air  upon  the  surface  of  a 
liquid,  the  more  rapidly  will  evaporation  go  on.  We  have 
already  mentioned  the  influence  of  pressure  upon  the  boiling 
of  liquids  in  §  142;  we  will  give  here  a  few  additional  il- 
lustrations. Ether  boils  when  heated  to  35°,  about  one 
and  a  half  degrees  below  the  heat  of  the  blood  in  our  bodies. 
If  we  place  some  of  it  in  a  vessel  under  the  receiver  of  an 
air-pump,  by  exhausting  the  air  we  can  so  lower  the  press- 
ure that  the  ether  will  boil  at  the  ordinary  temperature  of 
the  air  in  a  room. 


HEAT. 


277 


The  restraint  of  pressure  upon  boiling  is  very  strikingly 
shown  in  the  digester,  Fig.  240.  This  is  a  strong  boiler,  partly 
filled  with  water.  A  ther- 
mometer to  indicate  the 
temperature  of. the  water 
may  be  inserted  into  mer- 
cury contained  in  the  nar- 
row cup,  a.  Let,  now,  the 
boiler  be  heated  till  the  wa- 
ter boils,  the  air  being  left 
to  escape  by  the  stop-cock. 
If  the  stop -cock  be  shut 
and  we  continue  to  apply 
the  heat,  we  can  raise  the 
water  to  a  very  high  tem- 
perature without  its  boil- 
ing, because  of  the  press- 
ure of  the  condensed  steam 

upon  its  surface.  To  guard  against  the  danger  of  explo- 
sion a  safety-valve  is  provided,  having  a  weight  upon  it 
which  will  keep-it  shut  until  a  certain  amount  of  pressure 
accumulates,  and  then  it  is  forced  open,  letting  out  some 
of  the  steam.  An  apparatus  somewhat  after  this  plan, 
called  Papiri's  digester,  has  sometimes  been  used  in  cooking. 
The  great  heat  to  which  water  can  thus  be  raised  causes 
it  to  extract  the  nutritious  matter  from  bones  and  carti- 
lages, aiFording  material  for  soup  from  that  which  is  com- 
monly thrown  away. 

Distillation  is  a  process  whereby  a  liquid  converted  into 
vapor  is  again  condensed  into  a  liquid  by  cooling  it  in  a 
suitable  apparatus.  The  manner  of  conducting  the  opera- 
tion, and  its  application  to  the  purification  of  substances  and 
the  separation  of  liquids  possessing  different  boiling-points, 
will  be  explained  in  Part  II.  of  this  series — Chemistry. 

M2 


278  NATURAL  PHILOSOPHY. 

182.  Steam. — The  cloud  of  steam,  so  called,  which  you 
often  see  escaping  from  a  locomotive  is  not  really  steam. 
Steam  is  transparent  and  invisible.  This  may  be  rendered 
evident  by  watching  the  spout  of  a  tea-kettle  whence  steam 
is  issuing.  For  the  space  of  an  inch  or  so  from  the  end  of 
the  spout  nothing  is  visible ;  but,  at  a  greater  distance,  the 
steam  coming  in  contact  with  the  cooler  surrounding  air  is 
condensed  to  water,  and  this  mixture  of  water-drops  and 
steam  is  plainly  seen. 

The  Steam  -  Engine. — It  has  been  shown  in  §  138  that 
compressed  air,  in  an  air  -  gun  for  instance,  possesses 
great  power  by  virtue  of  its  elasticity.  Compressed 
steam  in  like  manner  exerts  enormous  force,  constituting, 
in  fact,  the  motive  power  of  the  steam-engine.  This 
machine,  complex  as  it  appears  to  the  casual  observer,  is 
not  so  difficult  of  comprehension  as  supposed  by  many; 
and,  Dr.  Arnott  justly  remarks,  any  one  "  who  can  under- 
stand a  common  pump  may  understand  a  steam-engine." 
It  is,  in  fact,  only  a  pump  in  which  the  fluid  drives  the 
piston  instead  of  the  piston  impelling  the  fluid ;  in  other 
words,  the  fluid  passing  through  the  cylinder  acts  as  the 
power  in  the  steam-engine  and  as  the  resistance  in  the 
pump.  We  cannot,  in  this  elementary  work,  enter  upon  an 
elaborate  description  of  the  steam-engine ;  but  we  wisli  to 
show  you  the  source  of  the  power  in  this  wonderful  inven- 
tion, and  for  this  purpose  shall  consider  only  the  simplest 
form. 

The  steam  is  generated  in  a  boiler,  having,  like  the  boiler 
of  Papin's  digester,  a  valve  with  a  weight  attached  to  it. 
This  valve  is  called  a  safety-valve,  because  when  the  steam 
has  reached  a  certain  degree  of  condensation  it  lifts  the 
valve,  and,  as  some  of  the  steam  escapes,  such  an  increase 
of  pressure  as  would  occasion  an  explosion  is  prevented. 
The  expansive  force  of  steam  in  a  boiler  is  estimated  in 


HEAT.  279 

pounds  on  the  weight  of  the  valve,  and  hence  the  common 
expression  that  there  are  so  many  "pounds  of  steam"  on. 
But  the  boiler  is  only  the  generator  of  steam,  and  it  re- 
mains to  show  how  the  steam  is  used  in  moving  machinery. 
This  is  done  by  allowing  the  steam  to  pass  from  the  boiler 
into  a  cylinder,  in  which  it  moves  a  piston  back  and  forth  by 
its  expansive  force.  The  manner  in  which  this  is  done  may 
be  made  clear  by  the  diagram,  Fig.  241.  Let  e  be  a  piston 
in  a  cylinder,/',  which  has  four  openings,  a,  b,  c, 
and  d.  Each  of  these  is  provided  with  a  valve, 
not  shown  in  the  diagram.  The  steam  is  sup- 
plied from  the  boiler  to  the  cylinder  through  a 
and  c,  and  makes  its  escape  from  b  and  d.  Sup- 
pose, now,  the  piston  be  near  the  bottom  of  the 
cylinder,  as  represented.  The  valve  at  a  is  opened 
that  steam  may  enter  to  push  up  the  piston,  and  f 
the  valve  at  b  shuts  that  the  steam  may  not  es- 
cape. At  the  same  time,  in  order  that  the  pressure 
may  be  removed  from  the  upper  surface  of  the 
piston,  the  valve  d  opens  that  the  steam  may  es- 
cape, and  c  shuts  that  none  may  enter.  When  the 


piston  is  to  be  forced  downward,  all  this  is  re-     a 

Fig.  241. 

versed — c  opens  to  admit  the  steam,  d  shuts  to  pre- 
vent its  escaping;  and  below,  b  is  opened  to  let  the  steam 
escape,  and  a  is  shut  to  prevent  any  from  entering.  This 
is  the  plan  of  what  is  called  the  high-pressure  engine.  The 
low-pressure  engine  differs  from  it  in  causing  the  steam,  as 
it  escapes  from  the  cylinder,  to  pass  into  water  to  be  con- 
densed. The  latter  requires  less  pressure  of  steam  to  work 
it,  and  is  therefore  the  safest.  The  manner  in  which  the 
motion  of  the  piston  is  made  to  drive  various  kinds  of  ma- 
chinery cannot  be  here  explained. 


280  NATURAL  PHILOSOPHY. 

QUESTIONS. 

164.  Describe  the  experiment  with  the  three  vessels  of  water,  and  the  in- 
ference from  it.  What  other  facts  sustain  this  inference?  How  did  Sir 
Humphry  Davy  prove  that  ice  contains  heat? — 165.  What  are  the  two 
theories  of  heat?  What  is  said  of  Count  Humford  and  his  experiment?  Give 
examples  of  the  conversion  of  motion  into  heat. — 166.  What  is  the  chief 
source  of  heat  for  the  earth  ?  What  is  said  of  the  heat  of  the  sun  itself? 
What  is  said  of  the  universal  influence  of  the  heat  of  the  sun  on  the  earth  ? 
What  of  the  heat  supplied  from  within  the  earth  itself?  What  is  said  of 
chemical  action  as  a  source  of  heat?  Give  examples  of  the  production 
of  heat  by  mechanical  action. — 167.  Show  the  expansive  influence  of  heat 
by  describing  the  experiment  with  a  bar  of  iron.  Give  familiar  examples 
of  this  expansion.  How  can  you  loosen  a  stopper  stuck  fast  in  a  bottle  ? 
Give  the  anecdote  about  the  Persia.  Give  the  statement  about  the  build- 
ing in  Paris.  Explain  how  the  unequal  expansion  of  metals  regulates  the 
length  of  pendulums.  What  is  a  gridiron  pendulum? — 168.  What  is  said 
of  the  expansion  of  liquids  by  heat  ?  How  may  the  influence  of  this  ex- 
pansion upon  specific  gravity  be  shown? — 169.  What  is  said  of  thermom- 
eters ? — 1 70.  What  of  the  invention  of  the  thermometer?  Explain  the  grad- 
uation of  Fahrenheit's  thermometer. — 171.  Give  the  plans  of  other  ther- 
mometers. Why  is  Celsius's  thermometer,  on  the  whole,  the  best  ?  Show 
how  to  convert  degrees  of  one  scale  into  those  of  another. — 172.  What  is 
said  of  the  expansion  of  gases  by  heat?  Describe  experiments  in  illustra- 
tion. What  is  said  of  balloons? — 173.  What  of  the  influence  of  heat  on 
the  atmosphere?  Give  examples  of  this  influence.  In  heating  apart- 
ments, why  is  heat  introduced  at  as  low  a  place  as  possible?  Explain  the 
draught  of  a  chimney.  Why  does  a  stove-pipe  generally  draw  better  than 
n  chimney? — 174.  Describe  the  experiment  with  the  candle  and  the  door. 
What  is  the  explanation  of  the  occurrence  of  wind  ?  Explain  the  land 
breeze.  Explain  the  sea  breeze. — 175.  How  are  winds  affected  by  the 
rotation  of  the  earth  ?  Show  why  the  prevailing  winds  at  the  equator 
are  northeast  and  southeast. — 176.  Mention  the  melting-points  of  various 
substances.  What  is  said  about  the  natural  state  of  water  and  other  sub- 
stances?— 177.  What  are  the  two  modes  of  changing  a  liquid  into  vapor? 
What  is  said  of  the  rapidity  of  evaporation  ? — 178.  What  is  said  of  moist- 
ure in  the  atmosphere  ?  What  influence  has  heat  upon  the  moisture  of 
the  air?  What  phenomena  illustrate  this? — 179.  What  becomes  of  the 
water  that  rises  in  the  air?  What  is  said  of  the  formation  of  fog  and  of 
clouds?  Mention  the  different  shapes  of  clouds  and  their  names.  What 


HEAT.  281 

is  said  of  the  influences  that  give  shape  to  clouds? — 180.  State  how  rain 
is  produced,  and  explain  Fig.  239.  How  are  snow  and  hail  formed  ? — 
181.  What  is  said  of  vaporization?  What  influence  has  pressure  upon 
the  formation  of  vapor?  Describe  the  experiment  with  ether  in  illus- 
tration. Describe  the  apparatus  represented  in  Fig.  240.  What  is  said 
of  Papin's  digester  ?  What  is  distillation  ? — 1 82.  What  is  said  of  steam  ? 
In  what  consists  the  power  of  the  steam-engine  ?  How  is  the  expansive 
force  of  the  steam  in  the  boiler  estimated  ?  Describe  the  manner  in  which 
steam  moves  a  piston  within  a  cylinder.  What  is  the  difference  between 
high  and  low  pressure  engines  ? 


CHAPTER  XVI. 
HEAT   (CONTINUED). 

183.  Communication  of  Heat. — Heat  has  a  constant  ten- 
dency to  an  equilibrium.  If,  therefore,  any  warm  substance 
be  in  the  neighborhood  of  a  cooler  one,  heat  passes  from 
the  former  to  the  latter.  This  communication  of  heat  oc- 
curs in  three  different  ways,  called  convection,  conduction, 
and  radiation.  We  will  speak  of  each  of  these  separately. 

Convection. — This  mode  of  diffusion  of  heat  operates  in 
those  substances  that  are  mobile — viz.,  liquids  and  aeriform 
substances.  We  have  already  alluded  to  examples  of  this 
mode  in  speaking  of  the  movements  which  heat  causes  in 
these  substances.  The  heat  accompanies  the  particles  which 
are  moved,  or  is  conveyed  along  with  them,  and  hence  the 
term  convection.  In  this  movement  the  heated  particles 
always  ascend,  for  the  reason  given  in  §  168.  Of  the  multi- 
tude of  examples  of  convection  we  will  present  but  a  few. 

The  upward  current  about  a  stove-pipe  furnishes  an  example  of  convec- 
tion, the  heat  generated  being  carried  upward  by  the  particles  of  this  cur- 
rent. This  being  so,  the  heat  of  a  stove  has  no  effect  upon  the  air  below 
it  by  convection,  though  it  does  by  radiation,  as  you  will  soon  learn.  Any 
hot  fluid  becomes  cool  chiefly  by  convection.  The  air  coming  in  contact 


282  NATUKAL   PHILOSOPHY. 

with  it,  taking  some  of  its  heat,  rises,  and  other  air  becomes  heated  in  turn, 
and  so  on  till  the  fluid  becomes  of  the  same  temperature  as  the  jiir,  an<| 
then  the  currents  of  air  cease.  The  liquid  cools  more  rapidly  by  stirring 
it,  because  the  air  is  brought  into  contact  with  a  greater  extent  of  surface, 
and  so  the  heat  is  conveyed  away  more  rapidly.  The  result  is  the  same 
whether  we  disturb  the  surface  by  stirring  it  or  by  blowing  upon  it.  In 
the  latter  case,  however,  the  effect  is  increased  by  causing  the  air  to  come 
more  rapidly  upon  the  disturbed  surface.  Thus  in  fanning,  it  is  the  rapidity 
with  which  the  air  is  brought  in  contact  with  the  surface  of  the  body  that 
causes  the  more  rapid  convection  of  heat  from  it.  Every  one  must  have 
observed  the  fact  that  a  buckwheat  cake  cools  much  more  quickly  than  a 
flour  or  rice  cake.  It  is  because  it  contains  so  many  pores  and  little  projec- 
tions, and  presents  a  much  larger  surface  to  the  heat-conveying  air  than 
the  smoother  and  more  solid  cakes.  Viscid  fluids,  such  as  molasses,  oil, 
chocolate  with  milk,  etc.,  when  heated  do  not  cool  so  readily  as  water,  be- 
cause their  particles  are  not  so  mobile,  and  therefore  heat  is  not  conveyed 
so  rapidly  upward. 

184.  Conduction. — In  tins  mode  of  diffusion  the  heat 
passes  through  or  among  the  particles  of  substances.  For 
example,  if  one  end  of  a  bar  of  iron  be  held  in  the  fire, 
it  travels  through  or  along  the  particles  to  the  other  end. 
The  gradual  progress  of  the  heat  may  be  seen  by  the  follow- 
ing simple  experiment :  Take  a  rod  of  iron  and  attach  to  it 
some  little  balls  of  wood  by  means  of  wax.  By  heating  one 
end  with  a  lamp  the  balls  will  drop  one  after  another,  as  the 
heat  passing  along  melts  the  wax  which  holds  them.  By 
making  this  experiment  with  two  rods  of  different  metals, 
as  shown  in  Fig.  242,  you  will  observe  that  heat  does  not 
travel  through  thorn  at  the  same  rate.  (§  185.) 


Fiir.  '24-J. 


HEAT. 


283 


Fig.  243. 


1 85.  Conductors  and  Non-Conductors. — Heat  is  conducted 
more  rapidly  through  some  substances  than  through  others: 
in  this  respect  there  is  great  variety.  That  this  is  the  case, 
even  among  those  which  are  reck- 
oned good  conductors,  is  shown 
by  the  experiment  represented  in 
Fig.  243.  Cones  of  the  same  size 
made  of  seven  different  substances 
— copper,  iron,  zinc,  tin,  lead,  mar- 
ble, and  brick — each  tipped  with  a 
little  wax,  are  placed  on  a  stove. 
The  wax  will  melt  on  the  copper 
cone  first,  showing  that  this  is  the 
best  conductor  of  all;  and  on  the 
brick  one  last,  showing  that  this  is 
the  poorest  conductor.  The  conducting  powers  of  the  rest 
are  according  to  the  order  in  which  we  have  mentioned  them. 

Another  way  of  making  this  comparison  is  to  substitute  small  pieces  of 
phosphorus  for  the  wax,  each  piece  taking  fire,  and  burning  with  a  brilliant 
flame  and  white  smoke  as  it  becomes  sufficiently  hgated  by  the  conduct- 
ing power  of  the  metallic  cone.  Care  should  be  taken  in  experimenting 
with  phosphorus  lest  it  be  ignited  by  the  warmth  of  the  fingers,  and  inflict 
very  serious  burns.  The  best  plan  is  to  cut  it  under  water,  and  to  use  very 
small  pieces. 

Those  substances  which  allow  heat  to  pass  through  them 
very  slowly  are  called  non-conductors.  .The  term,  though 
convenient,  is  not  strictly  correct,  for  there  are  no  sub- 
stances which  do  not  conduct  heat  in  some  degree.  Wood 
is  one  of  these  poor  conductors,  and  hence  wooden  handles 
are  put  upon  various  instruments  and  vessels  used  about 
fires,  such  as  the  soldering-irons  of  the  tin-man,  the  metal- 
lic teapot,  etc.  Since  cloth  is  a  non-conductor,  a  cloth 
holder  is  used  in  taking  off  the  tea-kettle,  and  in  using  the 
flat-iron.  Glass  is  so  poor  a  conductor  that  if  you  hold  a 


284  NATURAL   PHILOSOPHY. 

glass  rod  or  tube  in  the  flame  of  a  spirit-lamp  or  gas- 
burner,  and  heat  it  even  to  redness,  you  can  place  your 
finders  comparatively  near  to  the  heated  portion  with  im- 
punity. We  had  occasion  recently  to  bend  a  small  glass 
tube  in  this  way,  and  we  observed  some  water  in  it 
quite  near  the  heated  part,  which  remained  undisturbed 
through  the  process.  It  is  the  non-conducting  quality  of 
glass  that  makes  thick  pieces  so  liable  to  break  when  ex- 
posed to  any  sudden  change  of  temperature.  For  example, 
if  hot  water  be  poured  into  a  thick  glass  vessel,  the  inner 
Mirfaee  is  quickly  expanded;  but  the  outer  surface  does 
not  expand  so  rapidly  because  the  heat  is  not  readily 
conducted  through,  and  this  irregularity  in  expansion 
causes  a  fracture.  It  is  for  this  reason  that  the  flasks,  re- 
torts, etc.,  used  by  chemists  are  made  very  thin,  especially 

where  heat  is  to  be  ap- 
plied. 

180.  Davy's  Safety- 
Lamp.  —  One  of  the 
most  beautiful  applica- 
tions of  the  conduction 
of  heat  is  found  in  the 
safety-lamp  of  Sir  Hum- 
phry Davy,  an  inven- 
tion which  has  been 
the  means  of  savin  sf  the 
lives  of  multitudes  of  mi- 
ners. It  is  rep resen red 
in  Fig.  244,  which  pre- 
sents a  sectional  and  an 
external  view.  The  bot- 
tom part  contains  the  oil 
in  which  the  wick  lies 
Fig.244.  coiled;  when  lighted, the 


HEAT.  285 

flame  is  entirely  surrounded  by  wire  gauze,  as  shown  in 
the  engravings.  With  this  lamp  one  can  safely  go  into 
deep  mines  containing  the  most  explosive  gases.  All  that 
prevents  the  flame  within  from  setting  on  fire  the  gases 
without  is  the  cylinder  of  wire  gauze.  This,  being  a 
good  conductor,  carries  off  the  heat  of  the  flame  within  so 
rapidly  that  it  cannot  go  through  the  openings  as  flame, 
and  so  does  not  set  fire  to  the  gas  without.  The  facts 
upon  which  the  construction  of  this  lamp  was  based  were 
discovered  by  trying  many  experiments.  Among  them 
were  the  following  :  A  piece  of  wire  gauze  was  held  over  a 
candle  so  that  its  flame  struck  against  it.  The  smoke  is- 
sued above,  but  no  flame.  Then  a  stream  of  gas  was  al- 
lowed to  pass  through  the  gauze,  as  shown 
in  Fig.  245,  and  was  set  on  fire  above.  It 
burned  without  inflaming  the  gas  below. 
This  proved  that  flame  cannot  pass  through 
wire  gauze,  provided  the  meshes  are  not 
too  larufo.  The  safety-lamp  burns  well  in 
pure  air ;  but,  when  dangerous  gases  accu- 
mulate in  the  mines,  the  flame  is  extin- 
guished, and  this  serves  as  a  warning  to  Fi«- 245- 
the  miner.  The  terrific  explosions  constantly  occurring  in 
coal-mines  in  England  and  in  Pennsylvania  are  usually  the 
result  of  gross  carelessness  on  the  part  of  the  miners;  they 
open  the  lamp  to  light  a  pipe,  or  strike  a  match  for  some 
purpose,  instantly  igniting  the  explosive  gases.  Of  the 
nature  of  these  gases  you  will  learn  fully  in  Part  II.  of 
this  series  of  works.* 


*  As  in  the  case  of  many  other  inventions,  the  same  idea  was  originated 
and  put  to  practical  use  by  more  minds  than  one.  George  Stephenson,  who 
from  being  a  common  engine-might  in  a  colliery  rose  step  by  step  till  he  in- 
vented the  locomotive,  constructed  a  lamp  which  illustrates  in  another  way 
the  same  principle  — in  other  words,  he  invented  another  safety-lamp.  But 


286  NATURAL   PHILOSOPHY. 

187.  Relation  of  Density  to  Conduction. — Generally,  the 
more  dense  a  substance,  the  better  its  conduction  of  lie.it. 
Thus  metals  are  better  conductors  than  wood,  marble  than 
brick,  solids  than  liquids,  and  liquids  than  aeriform  sub- 
stances. A  good  illustration  of  the  difference  in  conduct- 
ing power  of  stone  and  brick  is  seen  in  the  melting  of  snow 
on  sidewalks.  If  a  light  snow  fall  in  the  spring,  after  the 
earth  has  become  somewhat  warm,  it  will  melt  on  the  stone 
walks  much  sooner  than  on  the  brick  ones.  This  will  be 
the  case  especially  if  the  snow  be  melted  chiefly  by  the 
\varmth  of  the  earth  without  the  agency  of  the  sun.  The 
explanation  is  obvions.  The  stone  is  a  better  conductor 
than  the  brick,  and  therefore  the  heat  of  the  earth  comes  up 
through  the  former  more  rapidly  than  through  the  latter. 

You  must  not  suppose,  however,  that  the  conducting 
power  of  the  metals  is  directly  proportioned  to  their  den- 
sity ;  platinum  is  much  denser  than  silver,  but  has  only  one 
twelfth  its  power  of  conducting  heat.  The  relative  posi- 
tion of  the  metals  in  this  respect  is  shown  in  the  follow- 
ing table,  the  figures  giving  the  approximate  conductivity 
compared  with  silver  taken  as  a  standard : 


Silver 100.0 

Copper 77.6 

Gold 53.2 

Brass 23.6 

Zinc 19.0 

Tin...  14.5 


Iron 11.9 

Steel 11.6 

Lead 8.5 

Platinum 8.4 

Palladium 6.3 

Bismuth...  1.8 


this  does  not  in  the  least  detract  from  the  glory  which  the  invention  has 
given  to  the  name  of  Davy,  for  each  acted  independently.  In  Davy's  case, 
it  is  to  be  remarked,  there  was  a  long  course  of  scientific  reasoning  and  in- 
vestigation which  led  him  at  length  to  the  invention,  the  record  of  which 
is  exceedingly  interesting.  No  invention  or  discovery  is  made  without 
thought,  though  accident  may  suggest  the  thought ;  but  here  is  an  inven- 
tion which,  without  any  suggestion  by  accident,  was  evolved  by  laborious 
and  long-continued  thought,  proceeding  step  by  step  to  its  conclusion. 


HEAT. 


287 


188.  Conduction  in  Liquids. — That  liquids  are  poor  con- 
ductors of  heat  may  be  shown  by  a  simple  experiment. 
Place  a  few  pieces  of  ice  at  the  bottom  of  a  test-tube  (a 
thin  glass  tube  closed  at  one  end),  and  add  water  until 
nearly  full.  By  heating  the  upper  portion  of  the  tube 
carefully,  the  water  may  be  boiled  without  a  particle  of 
the  ice  melting  in  the  bottom.  The  stand  for  holding  the 
tube  in  this  experiment  is  shown  in  Fig.  246. 


Fig.  246. 

If  the  heat  were  applied  at  the  lower  part  of  the  tube, 
the  ice  would  melt,  and  then  the  heat  would  be  diffused  by 
convection. 

189.  Air  as  a  Non-Conductor. — Heat  is  rapidly  diffused 
in  air  by  convection ;  but  this  takes  place  only  when 
the  air  is  free.  When  the  air  is  confined  in  spaces  or 
pores,  or  among  fibres,  heat  makes  its  way  through  it  very 
Blowly,  for  then  it  can  be  diffused  through  it  only  by  con- 


288  NATURAL   PHILOSOPHY. 

duction.  The  variety  of  ways  in  which  air  is  of  service  to 
us  as  a  non-conductor  is  almost  endless.  We  will  notice 
some  of  them. 

Double  Windows. — The  efficacy  of  double  windows  depends  upon  the 
air  imprisoned  between  them.  In  the  case  of  the  single  window  a  great 
deal  of  the  heat  inside  is  lost  in  this  way :  The  warm  air  of  the  room, 
coming  in  contact  with  the  window,  imparts  to  it  some  of  its  heat,  and,  be- 
ing thus  cooled  and  therefore  condensed,  passes  downward.  As  this  proc- 
ess goes  on  continually,  this  downward  current  along  the  window  is  con- 
stant. The  current  outside  is  in  the  opposite  direction.  The  heat  imparted 
to  the  window  is  taken  up  by  the  cold  air,  and  thus  becoming  warmer, 
it  passes  upward.  And  this  upward  current  outside  is  as  constant  as  the 
downward  current  inside.  Now  nearly  all  this  is  prevented  by  the  non- 
conducting quality  of  confined  air  in  the  case  of  double  windows.  If  a 
pane  were  removed  from  the  upper  part  of  the  inner  window,  and  another 
from  its  lower  part,  the  inner  window  would  be  of  little  use,  for  then  the 
heat  of  the  air  in  the  room  would  be  continually  diminished  by  convection, 
as  if  the  window  were  single.  The  warm  air  would  pass  in  at  the  upper 
opening,  and,  being  cooled,  pass  down  through  the  lower  one.* 

190.  Air  as  a  Non-Coiiductor  in  the  "Walls  of  Buildings. — 
The  spaces  included  between  the  outer  wall  of  a  building 
and  the  plastering  inside,  being  filled  with  imprisoned  air, 
prevent  the  heat  of  the  air  in  the  apartments  from  passing 
readily  through  the  wall.  A  house  built  of  brick  or  stone, 
with  the  plastering  placed  directly  upon  the  wall,  would 
be  kept  warm  with  greater  difficulty  in  winter,  because  the 
solid  wall  would  more  readily  carry  off  the  heat  to  the 
external  air.  For  a  similar  reason,  such  a  house  would  be 
very  warm  in  summer,  because  the  heat  of  the  sun  and  of 


*  The  author  once  adopted  a  contrivance  for  a  small  conservatory,  which 
he  wished  to  keep  warm  from  the  heat  of  an  adjoining  room.  In  each 
space  of  the  window-frames  were  inserted  two  panes  of  glass,  leaving  near- 
ly half  an  inch  of  space  between  them.  In  this  way  nearly  all  the  benefit 
of  double  windows  was  secured,  with  less  expense  and  a  less  cumbrous  ar- 
rangement. 


HEAT.  289 

the  external  air  would  be  so  rapidly  communicated  to  the 
air  of  the  house.  In  this  connection  we  may  mention  a  con- 
trivance to  prevent  the  spreading  of  fires  in  blocks  of  build- 
ings, which,  though  very  effectual,  is  seldom  used,  partly 
because  it  occasions  some  trouble  and  expense,  and  partly 
because  it  takes  up  a  little  room :  A  small  space  is  left  in 
the  division  wall  between  two  adjoining  houses  extending 
from  top  to  bottom,  and  containing,  of  course,  a  body  of 
imprisoned  air.  With  such  an  arrangement  the  interior 
of  one  house  may  be  entirely  consumed  without  transmit- 
ting sufficient  heat  through  the  imprisoned  air  to  fire  the 
other. 

191.  Pur,  Hair,  and  Feathers. — Animals  living  in  cold  cli- 
mates are  provided  with  suitable  coverings  for  their  pro- 
tection. Quadrupeds,  for  example,  are  covered  with  fur, 
and  birds  have  an  abundance  of  downy  feathers.  These 
coverings  have  no  warmth  in  themselves, though  in  common 
language  we  speak  of  them  as  being  warm.  They  are  simply 
non-conductors,  and  prevent  the  heat  generated  in  the  body 
of  the  animal  from  escaping  as  fast  as  it  otherwise  would. 
But  why  are  they  non-conductors?  It  is  partly  because 
the  substance  of  which  they  are  made  is  a  non-conductor, 
and  partly  because  that  great  non-conductor,  air,  is  impris- 
oned among  their  numberless  fibres.  But  if  the  fur  or  down 
were  compressed  into  a  thin  hard  plate  upon  the  animal, 
it  would  prove  of  far  less  service  as  a  protection  against 
cold.  Down  is  much  more  abundant  on  the  birds  of  cold 
climates  than  on  those  in  warmer  regions,  because  more  air 
can  be  confined  among  the  fibres  of  down  than  among  those 
of  common  feathers.  Quadrupeds  that  are  natives  of  warm 
climates  generally  have  hair  instead  of  fur.  When,  there- 
fore,^** horse  is  taken  to  a  cold  climate,  he  requires  in  win- 
ter the  protection  of  a  blanket ;  ana  the  ox  needs,  under  the 
same  circumstances,  to  be  bettor  housed.  The  elephant  be- 


290  NATURAL   PHILOSOPHY. 

ing  a  native  of  a  very  warm  climate,  has  scanty  and  coarse 
hairs.  Formerly  there  were  elephants  in  the  cold  regions 
of  Siberia,  as  has  been  ascertained  by  remains  found  there. 
But  the  elephant  of  Siberia  had  under  its  hair,  close  to  the 
skin,  a  fine  wool  to  serve  as  a  protection  against  the  cold. 
Animals  living  in  cold  climates  are  provided  with  cover- 
ings finer  in  fibre  in  the  cold  season  of  the  year,  to  give 
them  the  additional  protection  which  they  then  need.  And 
when  animals  with  a  furry  covering  are  carried  into  a  warm 
climate,  their  fur  becomes  coarse,  and  approximates  the 
condition  of  hair. 

192.  Clothing. — Man  has  no  covering  to  guard  him 
against  cold,  because  he  is  capable  of  contriving  clothing 
suitable  to  the  various  degrees  of  temperature  to  which  he 
may  be  exposed.  The  object  of  clothing  is  not  to  make  the 
body  warm,  but  to  keep  it  so.  The  heat  of  the  body  is 
continually  generated  within  itself,  and  under  all  circum- 
stances this  heat  is  maintained  quite  uniformly  at  98°  Fahr- 
enheit (36.6°  Centigrade).  This  is  a  much  higher  degree 
than  that  ordinarily  possessed  by  the  atmosphere.  We  are 
all  the  time,  then,  giving  off  heat  to  the  air  around  us,  ex- 
cept when  the  air  gets  up  to  98°.  We  are  comfortable  only 
when  we  are  giving  off  heat  to  a  considerable  amount,  for 
the  point  of  temperature  which  is  most  agreeable  when  we 
are  at  rest  is  70°  Fahrenheit  (21°  Centigrade),  or  a  little 
less — that  is,  28°  Fahrenheit  (or  15.6°  Centigrade)  lower — 
than  the  temperature  of  our  bodies.  When  the  tempera- 
ture is  below  this  we  need  extra  clothing.  In  making 
choice  of  clothing  for  various  degrees  of  temperature  we 
practically  apply  the  principles  just  developed.  Those  ar- 
ticles of  clothing  which  can  confine  or  entangle,  as  we  may 
say,  the  largest  quantity  of  air  among  their  fibre^-afe  the 
best  non-conductors,  or,  in  common  lan#««ge',  are  the  warm- 
est. And  loose  clothing  i». Banner  than  tight,  on  account 


HEAT.  291 

of  the  amount  of  air  between  the  clothing-  and  the  body. 
Thus  a  loose  glove  is  much  warmer  than  a  tight  one.  The 
same  general  fact  is  illustrated  by  the  coverings  of  straw 
placed  around  tender  trees  and  shrubs  in  winter.  It  is  the 
nir  confined  in  the  tubes  of  the  straw  which  makes  these 
coverings  so  effective  a  protection. 

Cocoons. — Many  insects  pass  through  their  pupa  or  transition  state  in 
cocoons.  When  this  is  done  during  warm  weather,  as  in  the  case  of  the 
silk- worm,  the  cocoon  is  simple.  But  when  the  pupa  state  lasts  through 
the  winter  special  provisions  are  made  in  the  arrangements  of  the  cocoon 
to  guard  the  insect  against  the  cold.  We  will  cite  as  an  example  the 
cocoon  of  one  of  our  largest  moths,  the  Cecropia.  This  cocoon,  fastened 
to  some  shrub,  keeps  its  inmate  secure  from,  the  rigors  of  the  winter  by  a 
very  beautiful  arrangement.  The  real  cocoon  is  similar  to  that  of  the  silk- 
worm ;  but  it  has  a  very  dense  air-tight  outer  covering,  and  the  space  be- 
tween these  two  coverings  of  the  pupa  is  filled  with  a  loose  substance, 
which  acts  the  part  of  a  blanket  for  the  insect. 

193.  Buds  of  Plants  in  "Winter. — In  the  latter  part  of 
summer  buds  are  formed  on  trees  and  shrubs,  and  these 
contain  the  germs  of  the  branches,  leaves,  and  flowers  which 
are  to  come  out  the  next  year.  These,  of  course,  must  be 
guarded  against  the  cold  of  winter,  and  it  is  done  very  much 
as  the  pupa  is  guarded  in  the  cocoon.  Each  bud  has  an 
air-tight  covering  of  scales  inside  of  which  is  a  soft  downy 
substance,  the  blanketing  of  the  bud.  In  these  coverings, 
which  have  been  called  by  some  one  the  "winter-cradles" 
of  the  buds,  the  infant  vegetation  of  another  year  rocks 
back  and  forth  in  the  wintry  winds  secure  from  the  cold, 
till  the  warm  sun  of  spring  wakes  its  hidden  life  into  ac- 
tivity. 

Snow  a  Protection  to  Plants.  —  Snow  acts  as  a  good  blanket  to  the 
earth,  keeping  its  warmth  from  escaping  into  the  cold  air.  This  is  because 
it  contains  a  quantity  of  air  mingled  with  its  feathery  crystals.  If  snow 
fall  early,  before  the  ground  and  the  plants  in  it  have  become  frozen,  it 
will  keep  them  from  freezing  through  the  winter,  provided  it  remain  dur- 


292  NATUIIAL    PHILOSOPHY. 

ing  all  that  time.  It  is  curious  to  observe  the  peculiar  arrangement  of  the 
snow  in  the  arctic  regions  for  the  preservation  of  vegetation.  First  in  the 
autumn  come  soft  light  snows  covering  up  the  grasses  and  heaths  and 
•willows.  Then,  as  winter  advances,  on  top  of  these  are  laid  the  denser 
snows,  making  a  compact,  stout  roof  over  the  lighter  snows  in  which  the 
scanty  but  precious  vegetation  of  those  regions  is  imbedded.  On  top  of 
this  roof  are  deposited  the  snows  of  spring.  As  these  melt  the  water  runs 
oft'  from  the  icy  roof  down  the  slopes,  leaving  untouched  the  plants  under- 
neath, which  lie  there  alike  secure  from  the  rush  of  waters  and  from  the 
nightly  frosts  until  the  season  is  sufficiently  advanced  to  bring  them  with 
safety  out  from  their  concealment.  Then  the  icy  roof  melts,  and  with  it 
the  light  snows  that  have  so  long  encircled  the  plants,  and  the  sun  wakes 
them  from  their  long  sleep  to  a  new  life. 

194.  Influence  of  the  Conduction  of  Heat  on  Sensation. — 
It'  you  place  your  hand  upon  fur  hanging  at  the  door  of  a 
fur-store,  it  does  not  feel  so  cold  as  the  wood  from  which  it 
hangs,  and  the  wood  does  not  feel  so  cold  as  the  iron  bar 
of  the  shutter  close  by.  Why  is  this,  when  these  sub- 
stances are  exposed  to  the  same  atmosphere,  and  really 
have  the  same  temperature?  It  is  because  the  iron  con- 
ducts the  heat  from  your  hand  more  readily  than  the  wood, 
and  the  wood  more  readily  than  the  fur.  For  a  similar 
reason  the  iron  handle  of  a  wooden  pump  feels  colder  than 
the  pump,  and  the  pump  colder  than  the  snow  around  it; 
and  the  rug  or  the  carpet  in  a  cold  room  will  not  feel  so 
cold  as  the  poker  and  the  hearth.  If  water  has  stood  long 
enough  in  a  room  to  acquire  the  same  temperature  as  the 
air,  your  hand  will  feel  colder  in  the  water  than  in  the  air, 
because  the  water  is  the  better  conductor.  So  much  for 
the  sensation  of  cold.  On  the  other  hand,  heated  sub- 
stances which  are  good  conductors  convey  the  sensation  of 
heat,  while  non-conductors  do  not.  As  they  receive  heat 
readily,  they  also  readily  impart  it.  For  this  reason,  with 
a  brisk  fire  the  hearth-stone  feels  very  hot,  while  the  rug 
before  the  fire  does  not. 


HEAT.  293 

195.  Radiation  of  Heat. — Every  substance  constantly  sends 
heat  into  space  in  straight  lines  in  every  direction.  These 
lines  are  radii,  and  hence  the  term  radiation  is  applied  to 
heat  diffused  in  this  way.  As  explained  in  §  165,  the  trans- 
fer of  heat  through  space  is  effected  by  the  imponderable 
fluid  called  ether.  That  the  sun  radiates  heat  in  all  direc- 
tions is  very  obvious.  The  same  can  be  perceived  in  the 
case  of  a  heated  iron  ball.  In  whatever  direction  you 
hold  your  hand,  above,  below,  or  laterally,  you  feel  the 
heat.  And  it  makes  no  difference  whether  the  ball,  be 
red-hot  or  not.  That  is,  heat  is  radiated  either  with  or 
without  light.  When  a  room  is  warmed  by  a  furnace,  it  is 
warmed  altogether  by  convection  ;  but  when  it  is  warmed 
by  a  fire,  either  in  a  fireplace  or  a  stove,  we  have  both  con- 
vection and  radiation.  The  heat  which  we  receive  from 
the  sun  comes  altogether  by  radiation  ;  it  travels  earth- 
ward with  about  the  same  velocity  as  light.  When  radi- 
ant heat  passes  through  air,  through  glass  lenses,  or  any 
other  medium,  they  are  not  warmed  by  it.  The  intensity 
of  radiant  heat  decreases  as  the  square  of  the  distance  from 
the  source — a  law  which  we  have  several  times  explained, 
first  with  respect  to  attraction,  and  secondly  as  to  sound. 
Now,  this  law  holds  good  also  for  light;  and  this  fact,  to- 
gether with  the  others  just  mentioned,  has  led  philosophers 
to  the  conclusion  that  heat  and  light  are  merely  different 
manifestations  of  the  same  force.  In  fact,  both  are  effects 
of  motion  (§  165).  All  surfaces  that  radiate  will  absorb 
equally  well  the  heat  radiated  upon  them.  All  rough 
and  dark  surfaces  both  absorb  and  radiate  freely;  but  all 
light-colored  and  polished  surfaces  do  both  slowly.  For 
this  reason  the  black,  rough  tea-kettle  is  well  fitted  to  heat 
water  in ;  but  it  is  not  fitted  to  retain  the  heat  in  the  wa- 
ter. On  the  other  hand,  the  bright,  polished  teapot  ab- 
sorbs heat  poorly,  but  retains  it  well. 

N 


294  NATURAL   PHILOSOPHY. 

196.  Reflection  of  Heat. — Radiant  heat  is  reflected  from 
polished  surfaces,  and,  as  in  the  case  of  motion  and  of 
sound,  §  155,  the  angles  of  incidence  and  reflection  are 
equal.  Some  interesting  experiments  in  relation  to  the  re- 
flection of  heat  can  be  tried  with  concave  metallic  mirrors. 
Thus,  if  we  take  two  such  mirrors,  Fig.  247,  and  place  in 
the  focus  of  one  a  thermometer  and  in  the  focus  of  the 
other  a  small  flask  of  hot  water,  or  a  heated  iron  ball,  the 
mercury  in  the  thermometer  will  rise,  although  the  mirrors 
may  be  many  feet  apart.  Observe  how  the  effect  is  pro- 
duced. Rays  of  heat  pass  from  the  flask  directly  towards 
the  thermometer,  as  represented  by  the  lines  in  the  figure ; 
but  that  the  effect  is  not  produced  by  these  can  be  proved 
by  removing  the  mirrors,  leaving  the  flask  and  thermome- 
ter in  the  same  positions.  When  the  experiment  is  tried 
in  this  way,  no  effect  is  produced  on  the  thermometer,  be- 
cause it  is  too  far  from  the  source  of  heat,  the  flask,  to  re- 
ceive any  perceptible  influence  in  this  way.  The  effect 
comes  from  the  rays  of  heat  which  pass  to  the  mirror  near 
the  flask,  and  are  reflected  to  the  other  mirror,  and  then 
are  reflected  upon  the  thermometer,  as  represented  by  the 
dotted  lines.  There  is  another  way,  besides  that  already 
mentioned,  of  showing  that  it  is  not  the  direct  rays  that 
produce  the  effect.  After  arranging  the  apparatus,  put  a 
screen  between  the  thermometer  and  the  mirror  near  it, 
and  the  effect  will  be  prevented  because  the  reflection  is 
cut  off.  If  a  piece  of  ice  be  substituted  for  the  flask  of  hot 
water,  the  thermometer  will  fall — an  effect  opposite  to  that 
produced  in  the  previous  experiment.  This  would  seem  to 
show  that  cold  is  radiated;  but  since  there  is  really  no  such 
thing  as  cold  (§  164),  the  effect  must  be  attributed  to  the 
radiation  of  heat  from  the  thermometer  to  the  ice.  If  a 
hot  ball  be  placed  in  the  focus  of  one  mirror  and  a  piece 
of  phosphorus  in  that  of  the  other,  as  represented  in 


HEAT. 


295 


Fig.  247,  the  phosphorus  will  be  ignited  even  when  the 
mirrors  are  twenty  or  more  feet  apart. 

The  reflection  of  heat  may  be  exhibited  very  prettily  by  a  much  sim- 
pler apparatus.  A  sheet  of  bright  gilt  paper  is  rolled  up  in  the  shape  of 
a  funnel,  with  the  metallic  side  inward.  Holding  the  larger  end  towards 
a  fire,  the  rays  of  heat  coming  from  the  fire  into  the  funnel  are  reflected 
towards  a  central  line,  and  so  pass  out  of  the  smaller  end  of  the  funnel. 
A  bit  of  phosphorus  or  a  lucifer-match  held  a  little  distance  from  this  end 
of  the  funnel  will  be  set  on  fire. 

197.  Formation  of  Dew. — Dew  is  forme'd  by  the  radiation 
of  heat  from  the  surface  of  the  earth.  The  earth  as  well 
as  the  sun  is  constantly  radiating  heat  into  space.  In  the 
daytime  the  earth  receives  a  great  deal  more  than  it  gives 
out ;  but  at  night  this  is  reversed,  and  the  earth  is  cooled. 
The  cooled  earth  condenses  the  atmospheric  moisture,  and 
this  moisture  is  deposited  in  the  form  of  little  drops  of 
water.  If  the  weather  be  very  cold,  this  is  frozen,  and 


296  NATURAL   PHILOSOPHY. 

then  we  have  frost  instead  of  dew.  You  observe  that  the 
dew  does  not/6///,  though  this  is  the  ordinary  expression. 
Its  formation  is  analogous  to  the  deposit  of  moisture  which 
we  so  often  witness  in  a  hot  day  in  summer  on  the  outside 
of  a  tumbler  containing  cold  water.  Just  as  the  cold  tum- 
bler condenses  the  moisture  in  the  air,  the  earth,  cooled  by 
radiation  at  night,  condenses  the  moisture  which  has  accu- 
mulated in  the  air  by  evaporation  during  the  heat  of  the  day. 
The  deposition  of  dew  and  frost  is  influenced  by  several 
circumstances.  Less  is  deposited  under  a  tree  than  away 
from  it,  because  all  the  heat  which  radiates  vertically  up- 
ward under  the  tree  is  reflected  back  again  by  it.  Hence 
the  efficacy  of  a  covering  over  plants  as  a  defence  against 
frost.  Clouds  operate  in  the  same  way,  and  for  this  reason 
no  dew  or  frost  is  deposited  in  a  cloudy  night.  Neither 
is  any  deposited  in  a  very  windy  night,  because  the  mov- 
ing air  promotes  evaporation,  and  thus  prevents  the  accu- 
mulation of  moisture. 

Dew  is  deposited  in  different  amounts  on  different  substances.  This  is 
owing  to  a  difference  in  radiation.  Grass  and  leaves  radiate  heat  better 
than  earth,  and  earth  better  than  stone ;  and,  therefore,  while  stones  and 
gravel-walks  may  be  dry  or  nearly  so,  the  loose  earth  may  be  moist  and 
the  grass  and  leaves  thoroughly  wet.  So  you  see  that  not  even  the  dew, 
plentiful  as  it  is,  is  wasted  by  the  Creator,  but  is  deposited  just  where  it  is 
ivanted  to  refresh  the  parched  earth  and  its  vegetation. 

Gideon's  Fleece. — If  you  spread  a  fleece  of  wool  upon  the  ground,  it  is 
so  poor  a  radiator  of  heat  that  no  dew  will  be  deposited  upon  it,  although 
the  dew  may  be  abundant  on  the  grass  and  leaves  in  its  neighborhood. 
But  this  was  reversed  in  the  case  of  Gideon's  fleece.  The  laws  of  nature 
were  set  aside,  and  the  fleece  was  wet  with  dew,  while  all  around  was  dry. 

198.  Dew-Point. — What  is  called  the  dew-point  of  the 
air  is  that  degree  of  temperature  to  which  any  s-ubstance 
must  be  reduced  in  order  that  dew  may  be  deposited  upon 
it.  This  depends  upon  the  amount  of  water  present  in 
the  atmosphere,  the  dew-point  rising  in  proportion  to  the 


HEAT.  297 

increase  of  moisture.  When  water  condenses  on  a  cold 
tumbler  in  a  hot  day,  there  is  much  more  water  in  the  air, 
and  the  dew-point  is  higher,  than  when  no  moisture  is  con- 
densed upon  the  tumbler.  Thus  after  a  very  hot  clear  day 
the  earth  need  not  be  much  cooled  to  produce  a  deposit  of 
dew,  because  the  air  has  become  so  highly  charged  with 
moisture  through  the  evaporation  of  the  earth  under  the 
hot  sun.  We  can  at  any  time  very  readily  ascertain  the 
dew-point.  Take  a  glass  of  water,  and,  placing  a  thermom- 
eter in  it,  drop  into  it  some  pieces  of  ice,  and  watch  the  out- 
side of  the  glass.  As  soon  as  it  begins  to  be  dimmed  with 
moisture,  read  the  thermometer  and  note  the  dew-point. 

On  clear  still  nights  water  is  sometimes  frozen  by  radia- 
tion, even  when  the  temperature  of  the  air  is  considerably 
above  the  freezing-point.  Advantage  is  taken  of  this  in 
the  tropical  climate  of  India  to  procure  ice  :  large  flat  and 
shallow  pans  containing  water  are  placed  on  straw  or  other 
non-conducting  material  and  sunk  slightly  into  the  earth  ; 
the  water  freezes  even  when  the  temperature  of  the  air  is 
10°  Centigrade. 

199.  Latent  Heat.  —  As  shown  by  the  experiments  in 
§  164,  our  sensations  do  not  inform  us  accurately  of  the 
amount  of  heat  in  any  substance.  The  same  is  also  true  of 
the  thermometer,  which  indicates  only  the  sensible  or  free 
heat.  A  great  deal  of  heat  may  be  locked  up,  as  we  may 
say,  in  the  substance,  that  can  be  brought  out  or  made 
free  by  a  change  of  state  in  that  substance.  This  heat 
thus  locked  up  is  called  latent  heat. 

Whether  a  substance  assumes  the  form  of  a  solid,  liquid, 
or  gas  depends  upon  the  amount  of  heat  latent  in  it.  If 
you  take  a  piece  of  ice  and  melt  it  in  a  vessel,  the  ice  and 
the  water  resulting  both  remain  at  0°  Centigrade  until  the 
ice  is  all  melted.  Yet  all  this  time  heat  is  being  commu- 
nicated to  the  ice  and  water.  What  becomes  of  it?  It  is 


298  NATURAL   PHILOSOPHY. 

all  taken  up  by  the  ice  as  it  changes  from  its  solid  to  its 
fluid  state,  and  becomes  latent  in  it.  In  fact,  every  particle 
of  ice  must  absorb  a  definite  amount  of  heat  in  order  to 
becoute  fluid.  If  water  be  heated  to  the  boiling  -  point 
(100°  Centigrade),  and  be  kept  boiling,  the  water  will  re- 
main at  that  point  till  it  is  all  vaporized.  All  this  time 
the  water  is  receiving  heat,  which,  instead  of  raising  its 
temperature,  is  becoming  latent  in  the  particles  as  they 
change  from  the  liquid  to  the  gaseous  state.  As  in  the 
change  from  the  solid  to  the  liquid  state,  so  in  this 
case,  every  particle  of  the  liquid  must  absorb  a  definite 
amount  of  heat  in  order  to  become  aeriform.  Whenever, 
therefore,  any  solid  substance  becomes  liquid,  or  liquid 
becomes  aeriform,  heat  is  absorbed  and  becomes  latent. 
On  the  other  hand,  whenever  any  aeriform  substance  be- 
comes liquid,  or  liquid  becomes  solid,  latent  heat  is  given 
out,  and  becomes  free  and  sensible.  The  freezing  of  water, 
then,  is  a  source  of  warmth  to  the  air  in  its  neighborhood 
— a  fact  which  is  practically  made  use  of  when  tubs  or 
pails  of  water  are  placed  in  conservatories  to  keep  plants 
from  freezing;  and  the  thawing  of  snow  and  ice  is  a  source 
of  cold,  as  is  exemplified  by  the  chilliness  of  the  air  occa- 
sioned by  this  process. 

200.  Recent  Theory  of  Latent  Heat.  —  The  expression 
latent  heat  was  introduced  into  the  science  of  phys- 
ics at  a  time  when  the  prevailing  doctrine  concern- 
ing the  nature  of  heat  admitted  its  existence  as  a  ma- 
terial substance ;  when  heat  disappeared  or  was  rendered 
latent,  it  was  supposed  to  enter  the  spaces  between  the 
molecules  of  matter;  and  when  it  was  rendered  sensible 
again,  it  was  supposed  to  be  squeezed  out,  as  it  were,  from 
the  infinitely  small  recesses  of  the  body.  Now,  however, 
since  heat  is  recognized  to  be  a  mode  of  motion,  the  expla- 
nation is  different;  when  a  solid  becomes  liquefied,  a  great 


HEAT.  299 

deal  of  heat  is  rendered  latent;  that  is,  the  heat  communi- 
cated to  the  solid  is  consumed  in  accomplishing  the  separa- 
tion of  the  molecules  and  in  overcoming  the  cohesion  nec- 
essary to  convert  it  into  a  liquid.  It  is  evident  that  if  the 
mobility  of  molecules  be  so  much  increased  as  occurs  in 
the  passage  of  solids  to  liquids,  the  heat  which  effects  this 
change  cannot  simultaneously  do  the  work  of  free  heat  or 
render  its  presence  appreciable  by  a  thermometer.  The  ex- 
pression latent  heat  is,  then,  liable  to  mislead  students;  yet 
it  has  taken  so  firm  a  hold  upon  the  language  of  heat-sci- 
ence, or  pyronomics,  as  it  is  sometimes  called,  that  we  can- 
not well  dispense  with  it.  You  should  remember,  however, 
that  heat  is  motion,  and  latent  heat  simply  motion  diverted 
to  do  other  work. 

Further  Illustrations. — This  subject  may  be  further  illustrated  by  some 
experiments,  easily  made,  the  results  of  which  are  apparently  quite  para- 
doxical. When  equal  quantities  of  hot  and  cold  water  are  mingled,  thk 
whole  becomes  lukewarm,  each  degree  lost  by  the  hot  water  becoming  a 
degree  gained  by  the  cold.  Mix,  for  example,  300  cubic  centimetres  of  wa- 
ter at  21°  Centigrade,  and  the  same  amount  of  water  at  54.°  Centigrade; 
the  temperature  of  the  mixture  will  be  the  mean  of  the  two — viz.,  37.5° 
Centigrade,  the  hot  water  losing  16.5°  and  the  cold  water  gaining  16.5°. 
In  like  manner,  a  mixture  of  equal  weights  of  water  having  the  tempera- 
ture of  0°  and  78°  respectively  would  yield  water  at  39°.  Now,  suppose 
we  repeat  the  last  experiment,  using  ice  at  0°  instead  of  water,  and  water 
of  78°;  you  might  expect  to  obtain  a  liquid  having  the  temperature  39° 
tis  before ;  but  this  is  not  the  result,  the  mixture  after  the  ice  has  melted 
will  have  only  the  temperature  of  0°  Centigrade.  What  has  become  of  the 
heat?  It  has  accomplished  the  work  of  converting  the  solid  ice  into  the 
mobile  liquid,  water,  and  is  not  now  capable  of  affecting  the  thermometer. 

The  refreshing  beverage  iced  tea  is  drunk  by  many  persons  in  warm 
weather.  Those  making  it  are  often  surprised  to  find  that  boiling-hot  tea 
poured  into  a  tumbler  of  ice  is  very  quickly  cooled  to  the  temperature  of 
the  ice  itself;  whereas  should  ice-cold  water  instead  of  ice  be  added,  a 
large  quantity  would  be  needed  to  cool  the  tea,  consequently  spoiling  it 
as  a  beverage.  The  reason  of  this,  however,  is  quite  plain  from  the  ex- 
planations just  given  of  latent  heat. 


300 


NATURAL    PHILOSOPHY. 


201.  Capacity  for  Heat. — The  more  heat  a  substance  can 
absorb  and  render  latent,  the  greater  its  capacity  for  heat, 
as  it  is  expressed.  Thus  water  lias  a  much  greater  ca- 
pacity for  heat  than  mercury.  This  can  be  proved  by 
various  experiments.  Take  two  vessels  just  alike,  and 
place  a  certain  quantity  of  water  in  one,  and  the  same 
quantity  of  mercury  in  the  other;  if  you  then  expose  them 
to  the  same  degree  of  heat,  it  will  take  much  longer  to 
raise  the  water  to  any  specified  temperature  than  the  mer- 
cury. Why  is  this,  when  they  are  both  receiving  the  same 
amount  of  heat  ?  It  is  because  the  water  renders  a  much 
larger  portion  of  the  heat  latent  than  the  mercury  does. 
We  can  reverse  this  experiment.  Take  these  same  vessels 
with  their  contents  raised  to  the  same  temperature,  as  in- 
dicated by  the  thermometer,  and  allow  them  to  cool  in 
the  air  side  by  side.  The  mercury  will  cool  faster  than 
the  water,  because  it  has  much  less  latent  heat  to  part 
with.  The  difference  in  capacity  for  heat  between  water, 
oil,  and  mercury  may  be  shown  by  the  experiment  repre- 
sented in  Fig.  248.  Put  one  hundred  grammes  of  water 


HEAT. 


301 


into  one  Florence  flask,  one  hundred  grammes  of  olive-oil 
into  another,  and  the  same  amount  of  mercury  into  a  third. 
Pleat  the  contents  of  each  flask  to  100°  Centigrade,  and  then 
place  them  in  funnels  filled  with  pounded  ice,  the  funnels 
resting  in  glass  jars  of  the  same  size.  Now,  in  cooling  these 
fluids  down  to  a  certain  point,  say  0°  Centigrade,  different 
amounts  of  the  ice  will  be  melted,  in  the  proportions  of  100 
:ind  50  and  3,  This  shows  the  proportions  of  latent  heat 
which  become  sensible  or  free  as  their  temperatures  are 
lowered. 

202.  Relation  of  Latent  Heat  to  Density.  —  The  more 
dense  a  substance  becomes,  the  less  its  capacity 'for  heat. 
The  heat  produced  by  hammer- 
ing iron  is  the  latent  heat  ren- 
dered free  by  condensation,  this 
lessening  the  capacity  of  the 
iron  for  heat.  The  same  thing 
can  be  better  illustrated  in  the 
condensation  of  a  very  com- 
pressible substance,  as  air.  Fig. 
249  represents  a  glass  syringe 
closed  at  one  end.  If  a  piece 
of  tinder,  or  a  little  bit  of  cot- 
ton wool  moistened  with  ether, 
be  placed  in  this  end,  and  the 
piston  be  forced  downward  very 
quickly,  the  tinder  or  the  ether 
will  be  set  on  fire.  This  is  be- 
cause the  compression  of  the 
air  lessens  its  capacity  for  heat 
so  much  that  a  great  deal  of 
its  latent  heat  is  made  sensi- 
ble or  free.  You  learned  in  §  131  that  the  atmosphere 
is  rarer  the  farther  you  go  from  the  earth.  It  is  very 


302  NATURAL   PHILOSOPHY. 

rare,  therefore,  on  the  summits  of  high  mountains.  This 

is  the  chief  reason  why  it  is  so  cold  there;  for  the  rarer 

the  air,  the  greater  its  capacity  for  heat,  and  the  more 
free  heat,  therefore,  can  it  render  latent. 

Clouds  and  Latent  Heat. — The  water  of  which  clouds  are  composed 
is  heavier  than  air.  Why,  then,  does  it  remain  suspended?  Why  is 
it  necessary  that  it  should  be  collected  into  drops  to  cause  its  descent? 
This  question  can  be  answered  by  looking  at  the  manner  in  which  clouds 
are  formed.  A  cloud,  as  stated  in  §  \  79,  is  made  up  of  minute  vesicles,  or 
bubbles,  containing  air.  Now,  the  air  in  these  bubbles  is  lighter  than  the 
air  surrounding  the  cloud,  because  it  is  warmer.  But  how  does  it  receive 
its  heat?  In  order  to  understand  this,  observe  from  what  the  bubble  is 
made.  It  is  made  from  the  water  which  was  in  the  air  in  a  state  of  vapor, 
or  aeriform,  for  this  is  the  state  of  water  in  the  atmosphere.  But  when  it 
forms  the  vesicle,  it  leaves  this  state  and  becomes  a  liquid,  for  the  wall  of 
the  vesicle  is  liquid.  Now,  in  passing  from  the  aeriform  to  the  liquid 
state  some  latent  heat  must  be  made  sensible.  This  sensible  heat  heats 
the  air  in  the  vesicle,  and  so  makes  it  like  a  heated  air-balloon.  Thus  all 
clouds  are  collections  of  innumerable  heated  air-balloons,  and  the  reason 
that  some  clouds  rise  higher  than  others  is  perhaps  that  their  vesicles  con- 
tain warmer  and  therefore  lighter  air. 

203.  Freezing  Mixtures. — The  intense  cold  produced  by 
these  mixtures  is  the  result  of  the  change  of  free  or  sensi- 
ble heat  into  latent.  For  example,  when  salt  and  snow 
are  mixed,  the  two  quickly  produce  a  liquid.  In  this 
sudden  change  of  a  solid  into  a  fluid  a  great  quantity 
of  heat  must  be  rendered  latent,  and  therefore  objects 
with  which  the  freezing  mixture  comes  in  contact  will 
suffer  a  great  loss  of  sensible  heat.  The  process  in  this 
instance  is  the  opposite  of  solidification.  A  portion  of 
the  snow,  after  melting  with  the  salt,  becomes  solid  ice. 
This  is  because  it  gives  up  its  sensible  or  free  heat  to  por- 
tions of  the  melting  snow,  which  are  causing  heat  to  be- 
come latent. 

The  low  temperatures  obtained  by  certain  freezing  mixt- 


HEAT.  303 

uves  are  remarkable :  for  example,  pulverized  Glauber*  s- 
salt,  moistened  with  hydrochloric  acid,  lowers  the  temper- 
ature from  10°  to  —17°  Centigrade — a  fall  of  27  degrees; 
a  mixture  of  one  part  by  weight  of  snow  and  one  of  dilute 
sulphuric  acid  lowers  the  temperature  from  — 7°  to  —51° 
Centigrade.  Still  greater  degrees  of  cold  can  be  obtained 
by  other  similar  means,  as  given  in  the  table  at  the  end  of 
this  chapter. 

204.  Cold  Produced  by  Evaporation. — If  you  pour  a  little 
ether  into  the  palm  of  your  hand,  it  will  rapidly  disappear 
in  vapor,  producing  a  sensation  of  great  cold.    This  sensation 
is  due  to  the  fact  that,  in  the  passage  of  the  liquid  to  the 
aeriform  state,  some  of  the  sensible  heat  of  your  hand  is  ab- 
stracted to  become  latent  in  the  vapor.     The  evaporation 
of  water  also  produces  cold,  though  not  so  decidedly  as 
ether,  because  its  change  into  vapor  is  not  so  rapid  at  or- 
dinary temperatures.    We  make  a  practical  use  of  the  evap- 
oration of  water  in  many  different  ways.    Thus  we  sprinkle 
water  in  a  hot  day  upon  the  floors  of  piazzas,  steps,  etc., 
that  much  of  the  sensible  heat  about  our  houses  may  be 
rendered  latent  by  the  evaporation.    For  the  same  purpose, 
in  hot  climates,  apartments  are  often  separated  from  each 
other  by  mere  curtains,  which  are  occasionally   sprinkled 
with  water.     In  like  manner,  the  inhabitants  of  such  cli- 
mates often  cool  their  beverages  by  wrapping  a  wet  cloth 
around  the  vessels  containing  them. 

Evaporation  is  an  important  remedy  in  many  cases  of  disease.  For  ex- 
ample, if  the  head  be  hot,  a  steady  application  of  a  wet  cloth  to  the  fore- 
head, though  a  simple  remedy,  is  often  effectual,  and  sometimes  is  very 
important.  Most  people  make  the  application  in  a  wrong  manner.  They 
put  on  several  thicknesses  of  cloth,  when  a  single  thickness  is  the  hest,  be- 
cause it  will  best  secure  the  evaporation,  which  is  the  cause  of  the  relief  af- 
forded. 

205.  Freezing  in  the  Midst  of  Boiling.  —  It  is  owing  to 


304 


NATURAL   PHILOSOPHY. 


the  quantity  of  heat  rendered  latent  by  evaporation  that 
water  can  be  frozen  in  the  midst  of  boiling  ether;  and, 
paradoxical  as  it  may  seem,  the  boiling  of  the  ether  causes 
the  freezing.  The  experiment  is  performed  in  this  way : 
Place  a  test-tube  or  a  little  thin  vial  with  water  in  it  in  the 
midst  of  some  ether  contained  in  a  shallow  vessel  under 
the  receiver  of  an  air-pump.  On  exhausting  the  air  the 
•ether  will  boil,  evaporation  taking  place  rapidly  because 
the  pressure  of  the  air  is  removed.  As  the  ether  passes 
into  vapor  it  extracts  so  much  free  beat  from  the  water 
that  it  is  cooled  down  to  the  freezing-point,  and  becomes 
solid.  Water  can  be  frozen  even  by  its.  own  evapora- 
tion. It  is  done  in  this  way :  Place  in  a  shallow  vessel,  b, 
Pig.  250,  a  little  water,  and  in  the  vessel  c  oil  of  vitriol  or 


sulphuric  acid ;  and  cover  the  whole  with  the  receiver,  a. 
When  the  air  is  exhausted  by  working  the  air-pump,  tho 
pressure  of  the  air  is  removed  from  the  water,  and  vapor 
rises  from  it  freely.  Since  the  sulphuric  acid  has  a  great 
attraction  for  water,  it  absorbs  this  vapor,  and  thus  vapor 
continually  rises  from  the  water;  this  proceeds  the  more 
rapidly  because  the  vapor  formed  is  absorbed,  instead  of 
remaining  to  cause  pressure  on  the  water.  By  tins  rapid 
formation  of  vapor,  requiring  a  great  quantity  of  heat  to 


HEAT.  305 

become  latent,  so  much  heat  is  at  length  abstracted  from 
the  water  remaining  that  it  freezes. 

20G.  Degree  of  Heat  Endurable  by  Man.— It  was  formerly  be- 
lieved that  the  human  body  could  not  endure  with  impunity,  even  for  a 
short  time,  a  much  higher  temperature  than  that  met  with  in  hot  climates. 
But  in  the  year  17GO  it  was  accidentally  discovered  that  a  much  higher 
temperature  than  this  could  be  endured.  At  that  time  an  insect  was  de- 
stroying the  grain  gathered  in  some  parts  of  France,  and  it  was  found  that 
if  the  grain  were  subjected  to  a  high  temperature,  the  insect  was  killed, 
and  yet  the  grain  was  not  injured.  In  trying  some  experiments  in  regard 
to  this  matter,  the  experimenters  wished  to  know  the  point  at  which  the 
thermometer  stood  in  a  large  oven.  A  girl  attending  the  oven  offered  to 
go  in  and  mark  the  thermometer.  She  did  so,  remaining  two  or  three 
minutes,  while  the  thermometer  was  at  127°  Centigrade— that  is,  27°  Cen- 
tigrade above  the  boiling-point  of  water.  As  she  experienced  no  great 
inconvenience  from  the  heat,  she  remained  ten  minutes  longer,  when  the 
thermometer  rose  to  142°  Centigrade.  These  facts  were  published,  and 
prompted  scientific  men  to  try  other  experiments.  In  England,  Dr.  For- 
dyce,  Sir  Charles  Blagden,  and  others,  went  into  rooms  heated  even  to 
115°  and  127°  Centigrade,  and  remained  long  enough  to  cook  eggs  and 
steaks,  and  yet  themselves  suffered  little  inconvenience.  The  pulse  was 
quickened,  the  perspiration  was  very  profuse,  but  the  heat  of  the  body,  as 
ascertained  by  putting  the  thermometer  under  the  tongue  the  moment 
they  came  out,  was  scarcely  raised  at  all.  The  air  in  which  they  remained 
roasted  eggs  quite  hard  in  twenty  minutes,  and,  applied  by  a  pair  of  bel- 
lows to  a  steak,  cooked  it  in  thirteen  minutes.  The  question  arises,  why- 
is  it  that  this  high  degree  of  heat  did  not  produce  a  more  injurious  effect 
upon  the  body  ?  One  reason  is  that  the  heat  of  the  air  in  the  immediate 
neighborhood  of  the  body  was  continually  reduced  by  the  evaporation  of 
the  free  perspiration,  sensible  heat  being  thus  converted  into  latent.  An- 
other reason  is  that  air  is  not  a  good  conductor,  and  therefore  did  not 
communicate  its  heat  readily  to  the  body.  Dr.  Fordyce  and  his  friends 
found  that  they  could  not  safely  touch  any  good  conductor,  such  as  metals, 
and  they  were  obliged  to  wear  upon  their  feet  some  non-conducting  sub- 
stance. 

207.  Formation  of  Ice. — Before  dismissing  the  subject  of 
heat,  we  must  notice  the  grand  exception  to  some  of  the 
operations  of  heat  in  the  formation  of  ice.  Heat  generally 


306  NATURAL   PHILOSOPHY. 

produces  expansion.  But  in  the  case  of  water  this  law  of 
expansion  is  set  aside,  and  the  reverse  is  established.  This 
is  the  case,  however,  only  within  a  small  range  of  tempera- 
ture—viz.,  from  the  freezing-point  up  the  scale  about  four 
degrees.  In  all  degrees  above  that  the  usual  expansion  by 
heat  takes  place.  The  exception  occurs  at  this  part  of  the 
scale  for  a  special  purpose — viz.,  in  order  that  water,  in  dis- 
tinction from  other  substances,  shall  become  more  bulky,  and 
therefore  lighter,  ichen  it  takes  the  solid  form. 

In  order  to  make  the  process  of  freezing  clear  to  you,  we 
•will  describe  it  as  it  ordinarily  occurs — that  is,  from  the  ac- 
tion of  cold  air  upon  the  surface  of  water.  The  uppermost 
layer  of  the  water  imparts  some  of  its  heat  to  the  air  in 
contact  with  it.  This  air  rises  and  colder  air  takes  its 
place,  which,  being  warmed,  rises  in  its  turn  to  make  way 
for  more  cold  air.  A  constant  current  of  warmed  air  rises, 
therefore,  from  the  water.  In  the  meantime  a  current  of 
a  different  character  forms  in  the  water  —  a  downward 
one.  As  fast  as  the  water  at  the  surface  parts  witli  heat 
to  the  air  it  falls,  other  warmer  water  taking  its  place,  to 
cool  in  its  turn  and  descend.  This  descent  of  the  cooled 
water  goes  on  regularly  until  a  portion  becomes  cooled 
down  to  +4°  Centigrade — that  is,  4°  above  the  freezing- 
point.  This  layer  does  not  sink,  but  remains  at  the  sur- 
face, for  it  is  lighter  than  the  warmer  water  below.  This 
is  because  the  law  that  heat  expands  matter  is  here  re- 
versed. Below  this  temperature  the  colder  the  water, 
the  lighter  it  is.  As  the  cooling  now  proceeds  as  before, 
the  cooled  water  at  the  surface  continually  increases. 
At  first  it  is  merely  a  single  layer  of  particles,  but  after 
a  while  quite  a  body  of  cold  water  rests  on  the  warmer 
water  below.  At  length  some  of  it  is  cooled  down  to  0°, 
the  freezing-point,  and  a  thin  film  of  ice  then  forms. 
The  state  of  things  just  at  this  stage  of  the  process  may 


HEAT.  307 

be  explained  by  the  aid  of  a 
simple  diagram,  Fig.  251.  Let 
the  line  a  represent  the  film  of 
ice.  The  space  between  a  and  b 
is  the  portion  of  water  cooled 
down  below  4°.  The  space  be- 
low b  is  occupied  by  the  water 
of  a  higher  temperature.  In  the 
space  between  a  and  b  the  cooler 

water  is  nearer  the  surface.  That  is,  from  the  line  b,  where 
the  water  is  exactly  at  4°,  the  water  lessens  in  tempera- 
ture as  you  ascend,  it  being  successively  3°,  2°,  1°,  etc.,  till, 
just  in  contact  with  the  film  of  ice,  a,  it  is  at  0°.  The  ice 
thickens  gradually  by  additions  below.  But  it  is  to  be  re- 
membered that  ice  is  a  good  non-conductor,  so  that  the 
very  first  layer  of  ice  makes  the  cooling  of  the  water  pro- 
ceed more  slowly  than  before.  And  the  thicker  the  ice 
becomes,  the  slower  the  cooling.  This  prevents  too  great 
a  formation  of  ice. 

208.  Why  the  Above  Exception  to  Expansion  by  Heat 
Exists. — That  we  may  understand  the  reasons  in  part  for 
the  grand  exception  to  the  general  law  of  expansion  by 
heat  above  illustrated,  let  us  examine  some  of  the  results 
if  the  exception  did  not  exist.  In  that  case  the  process 
of  freezing  would  be  as  follow* :  The  water  would  com- 
municate its  heat  from  the  surface  to  the  air,  as  before 
described,  and  there  would  be  a  constant  downward  cur- 
rent of  the  cooled  water.  When  any  portion  of  the  water 
became  cooled  by  the  air  down  to  0°,  it  would  become  ice, 
and  would  sink  to  the  bottom.  And  after  the  process  of 
freezing  had  once  begun,  there  would  be  a  continual  accu- 
mulation of  ice  at  the  bottom  so  long  as  the  air  remained 
cold  enough  to  cool  the  water  with  which  it  comes  in  con- 
tact down  to  0°. 


308  NATURAL  PHILOSOPHY. 

The  result  may  be  stated  in  general  thus:  Freezing 
would  not  begin  so  quickly  as  it  now  does;  but  when  once 
begun  it  would  prove  very  destructive.  It  would  not  be- 
gin so  soon,  because  the  whole  of  any  body  of  water  must 
be  cooled  down  to  a  temperature  very  close  to  0°  before  it 
could  begin.  This  would  not  take  long  in  ponds  and  streams 
where  the  water  is  shallow ;  all  shallow  bodies  of  water, 
then,  would  be  frozen  up  quite  early  in  the  winter;  and 
since  water  is  a  poor  conductor,  and  thawing  would  proceed 
from  above  downward,  some  of  them  would  not  be  thawed 
out  again  fully  till  quite  into  the  next  summer.  And  where 
the  water  is  quite  deep,  ice  would  at  length  begin  to  form, 
and  when  formed  it  would  be  exceedingly  slow  in  thawing. 
In  some  cases  it  would  never  thaw  with  such  a  body  of 
non-conducting  water  to  guard  it  against  the  warmth 
above.  It  is  easy  to  see  that  the  heat  of  spring  and 
summer  would  not  thaw  out  so  large  a  quantity  of  ice  as 
it  now  does.  The  reign  of  ice  and  snow  on  our  earth 
would  therefore  be  vastly  more  extensive  than  now,  and, 
what  is  worse,  it  would  extend  more  and  more  every  year. 
Under  such  circumstances  great  destruction  of  both  an- 
imal and  vegetable  life  would  result.  We  will  men- 
tion, however,  but  a  single  item,  for  it  would  occupy  too 
much  space  to  go  into  this  subject  more  fully.  In  the  wa- 
ter under  the  ice,  which  is  always  above  4°,  except  that 
wliieh  is  close  to  the  ice  during  its  formation,  there  is  a. 
'vast  amount  of  busy  life  which  would  be  destroyed  if  ice< 
were  formed  at  the  bottom,  chilling  all  the  water  above-., 

.209.  Force  of  Expansion  in  Ice. —  Since  ice  occupies^ 
seventh  more  room  than  the  water  from  which  it  is 
it  exerts  in  its  formation  an  expansive  force  whiehi  under 
various  circumstances  produces  varied  and  oftei*  remarka-- 
fole  results.    Of  the  numerous  experiments  which  have  been 
tried  to  -show  the  force  of  this  expansion  we-i\\iU;mention; 


HEAT.  309 

but  one  made  many  years  ago  in  Quebec.  A  bomb-shell 
was  filled  with  water  and  closed  with  an  iron  plug  which 
was  driven  in  with  great  force.  When  the  water  froze,  the 
plug  was  thrown  a  distance  of  more  than  450  feet  (150  me- 
tres) by  the  expansion  (Fig.  252).  This  expansion  is  some- 


~] 


Fig.  252. 

times  an  inconvenience  to  us,  as  in  bursting  water-pipes; 
but  besides  the  great  service  which  it  does  in  the  earth,  al- 
ready noticed,  it  is  of  advantage  also  in  loosening  the  soil, 
and  in  supplying  it  with  requisite  ingredients  from  the 
rocks  by  breaking  them  up  and  pulverizing  them  in  small 
quantities  from  year  to  year. 

210.  Scale  of  Temperature. — The  phenomena  effected  by 
heat  are  exceedingly  varied,  and  it  is  interesting  to  examine 
the  degrees  in  the  general  scale  of  temperature  at  which  cer- 
tain changes  ensue.  In  the- following  table,  taken  from  Dr. 
Arnott's  Elements  of  Physics,  only  a  few  facts  have  been 
selected  by  way  of  comparison.  The  figures  are  partly  from 
actual  observations  and  partly  calculated ;  they  are  given  in 
both  Centigrade  and  Fahrenheit  degrees.  The  extremely 
low  and  high  temperatures  are  only  approximations. 


310 


NATUKAL   PHILOSOPHY. 


TABLE   OF   HIGH   AND   LOW  TEMPERATURES. 

Degrees 
Centigrade. 


Estimated  "absolute  zero"  (Ganot) —273 

Greatest  artificial  cold  produced  by  nitrous  oxide 

and  carbon  disulphide  in  vacuo  (Natterer) —140 

Greatest  cold  from  a  bath  of  carbonic  acid  and 

ether  in  vacuo  (Faraday) — HO 

Liquefied  nitrous  oxide  freezes...  —101 

Liquefied  sulphurous  anhydride  freezes —77.2 

Greatest    natural   cold    observed   (in   Siberia   by 

Erinan) —57.7 

Liquefied  carbonic  acid  freezes —57.2 

Estimated  temperature  of  planetary  space  (Fourier).  —50. 

Mercury  freezes —39.4 

Mixture  of  equal  parts  of  sal  ammoniac  and  ice 

(Fahrenheit's  zero) —17.7 

Air    on    the   summit  of  Mont  Blanc,  February, 

1876,  3  P.M -12.2 

Ice  melts  (zero  of  Celsius  and  He'.uunur) 0 

Animal  heat  (the  "blood-heat  of  the  human  body  ")  +  36.6 

Highest  natural  temperature  observed  in  India. ...  +  60. 

Steamship  engine-room,  AVest  Indies 07.7 

Alcohol  boils 78.8 

Water  boils 100 

Tin  melts 227.7 

Bismuth  melts 2GO. 

Lead  melts 322.2 

Mercury  boils 343.3 

Black  heat 371.1 

Zinc  melts 411.5 

Antimony  melts 482.2 

lied  heat  visible  in  the  dark 537.7 

"           "         "        daylight 593.3 

Heat  of  a  common  fire 61G.1 

Bright-red  heat 648.8 

Silver  melts 1022.7 

Gold  melts .' 1249.9 

French  wrought  iron  melts 1500 

Hvdrogen  burned  in  air 1503.8 

Cast  iron  melts 1530 

English  wrought  iron  melts 1600 

Wind-furnace,  white  beat 1804.4 

Combustion  of  hydrogen  in  oxygen 3025.5 


HEAT.  311 

QUESTIONS. 

183.  What  is  said  of  the  communication  of  heat?  How  many  and  what 
are  the  modes  of  communication?  What  is  the  mode  called  convec- 
tion? Give  examples  of  convection. — 184.  What  is  the  conduction  of 
heat  ?  Describe  the  experiment  showing  the  gradual  progress  of  heat  in 
an  iron  rod. — 185.  Describe  the  experiments  showing  the  different  rates 
of  conductivity  of  different  substances.  What  is  said  of  non-conductors  of 
heat?  Give  the  examples  cited. — 186.  Explain  Davy's  safety-lamp.  What 
is  said  of  explosions  in  mines  ?  Give  what  is  stated  in  the  note  about  Ste- 
phenson  and  Davy. — 187.  What  is  said  of  the  influence  of  density  on' the 
conduction  of  heat?  Give  the  illustration  about  melting  snow. — 188.  State 
the  experiments  which  show  that  liquids  are  poor  conductors  of  heat. — 189. 
What  is  said  of  air  as  a  non-conductor  of  heat?  What  is  said  of  double 
windows? — 190.  What  is  said  of  arrangements  of  the  walls  of  buildings? 
What  of  an  arrangement  for  preventing  the  spreading  of  fires  in  blocks? 
— 191.  How  are  animals  in  very  cold  regions  protected  from  the  cold? 
What  is  it  in  their  coverings  that  affords  the  protection  ?  What  is  said  of 
the  coverings  of  quadrupeds  that  are  natives  of  warm  climates?  What  of 
the  elephants  whose  remains  are  found  in  Siberia?  What  changes  take 
place  in  the  coverings  of  animals  carried  from  a  cold  to  a  warm  climate, 
and  the  reverse? — 192.  Why  has  man  no  covering  against  the  cold?  Ex- 
plain the  object  of  clothing.  What  is  said  of  articles  of  clothing?  What 
of  loose  clothing?  What  of  straw  coverings  on  trees?  What  of  bricks 
compared  with  stones?  What  is  said  of  cocoons? — 193.  What  of  buds 
of  plants  in  winter?  What  of  snow  as  a  protection  of  plants?  State  the 
arrangement  of  snow  observed  in  the  arctic  regions. — 194.  State  in  full 
what  is  said  of  the  influence  of  the  conduction  of  heat  upon  sensation. — 
1 95.  What  is  meant  by  the  radiation  of  heat  ?  Give  examples  of  it.  What 
is  said  of  the  relations  of  heat  and  light?  What  is  said  of  the  relation  be- 
tween absorption  and  radiation? — 196,  What  of  the  reflection  of  heat? 
State  the  experiment  with  the  mirrors  and  the  thermometer  and  flask. 
Explain  the  experiment  with  the  ice.  Give  the  experiment  with  phospho- 
rus. Describe  the  experiment  with  a  cone  of  gilt  paper. — 197.  Explain  the 
formation  of  dew.  State  the  analogy  of  the  tumbler.  What  is  said  of  the 
circumstances  that  influence  the  deposition  of  dew  and  frost?  What  is 
said  of  different  substances  in  regard  to  the  deposition  of  dew  ?  What 
about  Gideon's  fleece? — 198.  What  is  the  dew-point?  How  can  you  ascer- 
tain it?  What  is  said  of  the  freezing  of  water  in  India? — 199.  What  is 
meant  by  sensible  heat,  and  what  by  latent  ?  Upon  what  does  the  form  of 


312  NATURAL   PHILOSOPHY. 

a  substance  depend?  State  in  full  what  is  said  of  the  melting  of  ice  and 
vaporization  of  water.  What  is  said  of  the  expression  latent  heat?  Ex- 
plain its  meaning.  Illustrate  by  reference  to  mixtures  of  cold  water  with 
hot,  and  of  ice  with  hot  water.  What  is  said  of  iced  tea  ? — 200.  What  is 
said  of  capacity  for  heat?  Describe  and  explain  the  experiment  with  wa- 
ter, oil,  and  mercury  of  the  same  temperature. — 201.  What  is  the  relation 
of  heat  to  density  ?  Give  the  illustrations.  What  is  the  reason  that  the  air 
is  so  cold  on  great  heights  ? — 202.  State  in  full  what  is  said  of  latent  heat 
in  reference  to  clouds. — 203.  Explain  the  operation  of  freezing  mixtures. 
— 204.  State  the  examples  of  the  production  of  cold  by  evaporation. — 205. 
Describe  and  explain  the  experiment  with  ether  and  an  air-pump ;  also 
that  with  water  and  sulphuric  acid  in  vacuo. — 206.  Give  the  facts  stated  in 
regard  to  the  degree  of  heat  which  man  can  endure.  Give  the  reasons  why 
the  heat  did  not  produce  a  greater  effect  in  these  cases. — 207.  What  effect 
does  heat  produce  upon  the  bulk  of  substances  ?  What  is  said  of  water  as 
an  exception  ?  Describe  the  process  of  freezing  as  illustrated  by  the  diagram. 
— 208.  What  would  be  the  process  if  the  exception  did  not  exist  ?  State 
what  would  be  the  results. — 200.  What  is  said  of  the  force  of  expansion  in 
ice?  State  some  of  the  benefits  which  come  from  this  expansion? — 210. 
Quote  from  the  table  some  of  the  remarkable  temperatures  at  which  certain, 
phenomena  occur. 


CHAPTER  XVII. 

LIGHT. 

211.  Nature  of  Light. — The  exact  nature  of  light  is  not 
known.  There  are  two  suppositions  in  regard  to  it.  One 
is  that  of  Sir  Isaac  Newton,  called  the  theory  of  emission. 
According  to  this,  light  is  a  material  substance,  but  so  sub- 
tile as  to  possess  no  weight  and  to  be  impalpable;  and, 
being  thrown  off  in  all  directions  from  the  sun,  it  passes 
with  inconceivable  rapidity  through  the  atmosphere,  and 
even  through  substances  of  great  density.  The  minute 
particles  of  this  substance  striking  the  eye  produce  the 
sensation  of  light,  just  as  particles  thrown  off  by  an  odor- 
ous body  affect  the  organs  of  smell. 


LIGHT.  313 

The  other  supposition,  first  definitely  advanced  by  Huy- 
ghens,  is  known  as  the  undulatory  theory.  The  advo- 
cates of  this,  which  is  now  quite  generally  received,  believe 
light  to  consist  of  undulations,  waves,  or  vibrations  in  an 
imponderable  fluid  or  ether  which  is  supposed  to  exist  ev- 
erywhere, pervading  all  space  and  every  substance.  You 
perceive  here  an  analogy  to  sound,  but  the  vibrations  of 
sound  are  in  a  palpable  medium,  and  those  of  light  (as  well 
as  of  heat)  are  in  an  imponderable  ether. 

The  vibrations  of  light,  moreover,  differ  essentially  from 
those  of  sound  in  the  manner  of  their  transmission.  You 
have  seen  in  §  148  that  the  pulses  or  vibrations  of  air  caus- 
ing sound  are  in  the  direction  of  the  line  of  transmission, 
whereas  in  light  the  vibrations  are  at  right  angles  to  this 
direction.  This  kind  of  wave  motion  is  shown  in  Fig.  253, 


Fig.  253. 

in  which  the  white  dots  represent  particles  of  ether,  and 
the  light  is  supposed  to  pass  in  the  direction  A  B.  Each 
particle  in  succession  makes  a  to-and-fro  motion  in  the  di- 
rection V  b",f'f",  etc.,  coming  finally  to  rest  on  the  line 
A  B.  This  peculiar  wave  motion  may  be  illustrated  by 
shaking  a  stiff  cord  or  rope  from  one  end,  when  waves  will 
appear  to  run  along  the  rope,  though,  of  course,  there  is  no 
actual  transfer  of  the  particles  of  which  the  rope  is  made. 
Referring  again  to  Fig.  253,  the  distance  V  c'  is  called  a 
wave-length,  and  the  distance  V  I"  or  c'  c"  the  amplitude 
of  the  wave.  These  distances  differ  in  different  kinds  of 
light  as  will  be  explained  in  the  latter  part  of  this  chapter. 


314  NATURAL   PHILOSOPHY. 

The  two  theories  just  named  correspond  to  the  two  the- 
ories of  the  nature  of  heat  as  explained  in  §165:  heat  is 
supposed  to  be  due  to  the  vibrations  of  the  universal  ether ; 
and  light  being  ascribed  to  the  same,  it  only  remains  to  dis- 
tinguish the  two.  The  difference  between  heat  and  light 
is  believed  to  depend  on  the  velocity  of  the  vibrations  of 
the  ethereal  substance,  those  of  light  being  far  more  rapid 
than  those  of  heat.  In  both  cases  the  vibrations  of  the 
ether  are  believed  to  be  excited  by  the  motions  of  the  mol- 
ecules of  the  substances  generating  the  heat  or  light.  This 
theory  assists  in  explaining  what  is  known  as  the  converti- 
bility of  heat  into  light.  Any  solid  substance  heated  to  a 
sufficiently  high  temperature  emits  light.  Of  this  we  have 
numerous  examples:  a  blacksmith  hammers  a  piece  of  iron 
until  it  becomes  at  first  warm,  then  hot,  and  eventually 
red-hot  /  in  other  words,  until  it  emits  light.  The  frequent 
association  of  light  with  heat  in  the  phenomenon  of  combus- 
tion is  another  example.  In  these  cases  the  particles  or 
molecules  of  the  substances  are  set  in  motion,  and  we  ob- 
serve heat  with  or  without  light  according  to  the  rapidity 
of  their  vibrations :  when  circumstances  quicken  immense- 
ly those  vibrations  which  produce  the  effect  we  call  heat, 
they  then  impart  the  sensation  of  light. 

212.  Sources  of  Light. — Any  substance  capable  of  com- 
municating light- vibrations  to  the  surrounding  ether  is 
said  to  be  luminous.  Our  chief  source  of  light  is  the  sun. 
What  particular  conditions  exist  in  the  sun  enabling  it  to 
send  out  such  prodigious  quantities  of  heat  and  light  dur- 
ing such  enormous  periods  of  time  is  not  well  understood. 
The  light  is  probably  the  result  of  intense  heat,  but  wheth- 
er this  heat  arises  from  combustion  similar  to  that  on  the 
earth  or  from  some  other  causes  has  not  been  determined. 
Other  sources  of  light  are  the  stars  which,  like  the  sun,  are 
self-luminous.  The  moon,  however,  as  you  know,  shines  by 


LIGHT.  315 

reflected  light,  that  of  the  sun.  Chemical  action  is  a  very 
important  source  of  light,  all  combustion  is  merely  intense 
chemical  action  accompanied  by  heat  and  light,  as  will  be 
shown  in  Part  II.  of  this  series — Chemistry.  Hydrogen  gas 
burns  with  a  very  feeble  non-illuminating  flame;  but  if 
you  introduce  a  platinum  wire,  it  will  glow  or  become  in- 
candescent on  account  of  the  intense  heat.  The  illuminat- 
ing power  of  ordinary  kerosene  or  oil  lamps  is  due  to  the 
incandescent  solid  particles  floating  in  the  flames. 

As  given  in  the  table  on  page  310,  the  temperature  at 
which  red  heat  becomes  visible  in  the  dark  is  about  538° 
Centigrade  (  =  1000°  Fahrenheit),  and  in  daylight  593°  Cen- 
tigrade (=1100°  Fahrenheit);  but  these  figures  are  only 
approximate. 

Electricity  is  a  source  of  light ;  this  is  seen  in  the  light- 
ning flash,  the  spark  of  a  frictional  machine,  and  the  effects 
produced  by  a  galvanic  battery  (Chapters  XVIII.  and 
XIX.). 

There  are  many  other  inferior  sources  of  light:  the  phos- 
phorescence of  decaying  animal  and  vegetable  matter,  the 
so-called  phosphorescence  of  certain  bodies  which  appear 
luminous  after  exposure  to  the  sun's  rays,  and  the  phospho- 
rescence of  certain  living  animals,  as  fire-flies,  glow-worms,, 
and  animalcula  in  the  sea. 

213.  Opaque  and  Transparent  Bodies. — Bodies  which  per- 
mit the  free  passage  of  light  through  them  are  said  to  be 
transparent ;  those  which  obstruct  the  rays  of  light  are 
called  opaque.  Intermediate  between  these  are  certain 
bodies  which  allow  light  to  pass  through  them  dimly: 
these  are  said  to  be  translucent.  Transparent  and  opaque 
are  relative  terms,  some  substances  usually  called  opaque 
becoming  translucent  when  made  excessively  thin.  Of  this 
we  have  an  example  in  gold,  which  transmits  a  greenish 
light  when  beaten  out  into  very  thin  leaf. 


316  NATUBAL   PHILOSOPHY. 

Light,  like  heat  and  sound,  radiates  in  straight  lines  in 
all  directions  from  its  source.  We  can  see  this  to  be  true 
by  admitting  rays  of  light  into  a  darkened  room  through 
small  openings  in  the  shutters,  the  rays  making  straight 
lines  across  the  darkness,  as  shown  by  the  motes  flying 
in  the  air.  The  fact  is  recognized  by  the  marksman  in 
taking  aim,  and  by  the  engineer  in  making  his  levels. 
The  carpenter  acts  upon  it  when  he  tests  the  smooth- 
ness of  any  surface  by  letting  the  light  pass  along  over 
it  to  his  eye. 

The  manner  in  which  opaque  bodies  cast  shadows  also  il- 
lustrates this  fact,  as  shown  in  Fig.  254;  light  radiating  from 


Fig.  254. 

the  point  S  and  striking  the  opaque  body  forms  a  conical 
shadow.  The  shape  of  the  shadow  is  determined  by  draw 
ing  straight  lines  from  the  point  S  to  either  edge  of  the  in- 
tervening substance,  and  continuing  these  lines  indefinitely. 
214.  Intensity  of  Light. — As  light  passes  in  all  directions 
from  any  body  or  point,  the  farther  we  go  from  its  source, 
the  less  bright  will  the  light  be.  The  farther  we  trace  any 
two  rays  of  light  from  their  source,  the  farther  are  they 
separated  from  each  other,  and  that  which  is  true  of  any 
two  rays  is  true  of  all  the  rays.  It  follows  that  the  farther 
any  surface  is  removed  from  a  source  of  light,  the  less  light 
will  fall  upon  it.  This  decrease  of  light  in  proportion  to 
distance  is  perfectly  regular,  being  as  the  square  of  the  dis- 
tance ;  or,  in  other  words,  the  intensity  of  light  is  inversely 
as  the  square  of  the  distance  (§  28).  To  illustrate  this  ex- 
perimentally, place  a  screen,  a  candle,  and  a  square  piece  of 
pasteboard  between  them  at  one  foot  from  each,  as  shown 


LIGHT. 


317 


Fig.  255. 


in  Fig.  255.  The  shadow  on 
the  screen  covers  a  space  four 
times  as  large  as  the  paste- 
board; that  is,  the  light  that 
shines  on  the  pasteboard,  if  al- 
lowed to  pass  on  to  the  screen, 
would  be  diffused  over  four 
times  the  space,  and  therefore 
would  have  only  one  quarter 
of  the  intensity.  So  if  the  screen  be  placed  at  twice  the 
distance  from  the  pasteboard  (Fig.  256),  the  shadow  will 

cover  a  space  nine  times 
as  large,  and  therefore 
the  light  there  would 
have  one  ninth  of  the 
intensity.  Again,  it  is 
seen  by  Fig.  257  that  if 
the  screen  be  placed  at 
the  distance  of  three  feet, 
Fi»-256<  the  intensity  of  the  light 

is  one  sixteenth  of  that  which  it  is  at  the  pasteboard. 
While  the  dis- 
tances, therefore, 
are  1,  2,  3,  4,  etc., 
the  intensity  of  the 
light  is  inversely  as 
the  numbers  1, 4,  9, 
16, etc.;  that  is,  in- 
versely as  the  square 
of  the  distance. 


Fig.  257. 


Advantage  is  taken  of  this  regularity  to  compare  the  intensities  of  lights 
from  different  sources.  Fig.  258  represents  a  very  simple  method  of  test- 
ing the  comparative  strength  of  two  lights.  C  D  is  a  white  surface  in  front 
of  which  the  small  rod,  s,  is  fastened.  When  a  light  is  placed  at  /  and  an- 

o 


318 


NATUEAL   PHILOSOPHY. 


Fig.  25S. 


other  at  L,  two  shadows  are  termed  upon  the  white  surface.  Now,  if  the  two 
lights  be  of  equal  intensity,  both  the  shadows  will  be  equally  dark  when  the 
lights  are  at  the  same  distance;  but  if  the  light  L  be  the  brighter,  the  shad- 
ow a  will  be  darker  than  b ;  and  to  obtain  shadows  of  equal  darkness,  the 
stronger  light  must  be  moved  back  from  the  rod  s.  This  being  done,  and 
the  distance  of  each  light  measured,  't  is  easy  to  calculate  their  relative  in- 
tensities. 

Instruments  called  photometers,  from  two  Greek  words  signifying  "  light- 
measurer,"  are  employed  to  examine  the  strength  of  the  flame  of  coal  gas. 
These  instruments  operate  on  a  principle  similar  to  that  just  described,  and 
the  gas  light  is  compared  with  that  of  candles  of  standard  size  and  weight. 

215.  Velocity  of  Light.— The  velocity  of  light  is  so  great 
vhat  within  ordinary  distances  it  may  be  considered  as  in- 
stantaneous. Thus  when  we  measure  the  distance  of  a 
cannon  by  the  difference  between  the  time  of  its  flash  and 
the  report,  we  do  not  allow  for  the  time  consumed  by  the 
light  in  its  passage  to  the  eye.  But  when  we  look  at  ob- 
jects as  distant  as  the  sun  and  other  heavenly  bodies,  we 
allow  in  our  calculations  for  the  time  consumed  by  the  pas- 
sage of  light.  It  takes  light  eight  and  one  quarter  minutes 
to  travel  from  the  sun  to  us,  a  distance  of  ninety-two  mill- 
ions of  miles.  "With  the  telescope  stars  have  been  seen 
which  are  ascertained  to  be  at  such  a  distance  that  it  re- 
quires over  ten  years  for  their  light  to  reach  the  earth. 


LIGHT.  319 

Others  have  been  seen  which  are  much  farther  off,  but 
their  distances  have  not  been  absolutely  ascertained ;  some 
are  supposed  to  be  so  remote  that  the  light  occupies  many 
centuries  in  its  passage  to  the  eye  of  the  astronomer ! 

Roemer's  Observations. — The  velocity  of  light  was  first  determined  by 
Roemer,  a  Danish  astronomer,  in  1676.  It  was  done  by  calculations  and 
observations  of  the  eclipse  of  one  of  Jupiter's  moons.  After  making  the 
calculation  of  the  time  it  would  take  for  the  satellite  to  pass  through  the 
shadow  of  the  planet,  he  observed  its  passage,  and  found  that  it  did  not 
emerge  from  the  shadow  so  soon  as  his  calculation  required  by  fifteen 
seconds.  What  was  the  difficulty  ?  If  the  earth  had  remained  in  one  spot 
from  the  beginning  to  the  end  of  the  passage  of  the  satellite,  the  observation 
would  have  agreed  exactly  with  the  calculation.  But  the  earth  had  moved 
in  its  orbit  during  this  time  (about  forty-two  hours  and  a  half)  the  immense 
distance  of  2,880,000  miles.  The  light  of  the  emerging  satellite  therefore 
had  to  travel  over  this  additional  distance  to  overtake  the  earth,  and  it 
took  fifteen  seconds  to  do  it.  If  we  divide,  then,  this  distance  by  15,  we 
get  the  distance  which  light  travels  in  a  second,  which  is  192,000  miles. 
All  this  can  be  made  clear  by  the  diagram  Fig.  259.  Let  S  be  the  sun, 
J  Jupiter,  and  C  one 
of  its  moons  emerg- 
ing from  its  shadow. 
Let  A  be  the  earth 
as  it  is  when  the 
eclipse  of  Jupiter's 
moon  begins.  When 
it  emerges,  the  earth 
has  passed  to  B,  and 
the  light  from  the  sat- 
ellite  has  to  travel  as 
much  farther  to  reach  it  now  as  B  C  is  longer  than  A  C.  Roerner  made 
other  observations  with  the  earth  at  some  other  parts  of  her  orbit  with  sim- 
ilar results. 

216.  Reflection  of  Light. — Light,  like  sound  and  heat,  is 
reflected  in  straight  lines  when  it  strikes  upon  any  resist- 
ing substance.  This  is  evident  when  it  strikes  any  smooth 
and  plane  surface.  And  it  is  true  of  light,  as  it  is  of  heat, 


320 


NATURAL   PHILOSOPHY. 


that  the  angles  of  incidence 
and  reflection  are  equal. 
Thus  if  s  s'  (Fig.  260)  be  a 
reflecting  surface,  and  n  p 
a  line  perpendicular  to  it, 
then  a  ray  of  light,  /  w, 
will  be  reflected  in  the  line 
n  d,  and  the  angle  of  inci- 
dence,/wjo,  will  be  equal  to 
the  angle  of  reflection,  pnd. 
The  various  objects 

around  ns  are  made  visible  by  the  light  reflected  from 
them.  Every  point  of  each  surface  we  see  reflects  rays  or 
vibrations  of  light  to  our  eyes.  Thus  when  we  see  a  per- 
son, rays  of  light  are  reflected  into  our  eyes  from  every  part 
of  him.  These  rays  form  an  image  of  him  in  the  back  part 
of  each  eye,  and  it  is  by  this  image  that  we  see  him,  as  will 
be  explained  more  fully  in  another  part  of  this  chapter. 
Reflected  light  is  painting  the  images  of  objects  in  the  eye 
every  moment  in  great  abundance  and  variety.  If  a  speak- 
er have  an  audience  of  a  thousand  persons,  each  one  looking 
at  him,  his  image  is  at  the  same  time  in  two  thousand  eyes, 
and  in  each  of  these  two  thousand  images  every  motion  and 
every  changing  expression  is  faithfully  depicted. 

217.  Mirrors. — That  reflected  light  does  thus  form  im- 
ages of  objects  is  seen  in  the  common  mirror.  The  image 
of  any  object  formed  is  produced  by  the  light  reflected 
from  th'at  object  to  the  glass.  Then  in  seeing  the  image 
light  is  reflected  from  it  into  the  eye,  there  to  form  a  sim- 
ilar image,  though  of  much  less  size.  By  using  two  or 
more  mirrors  the  reflections  of  the  image  can  be  multiplied, 
and  by  certain  arrangements  to  a  very  great  extent.  When 
you  look  at  the  reflection  of  any  object  in  a  mirror,  the  im- 
age appears  to  be  at  the  same  distance  beyond  the  surface 


LIGHT. 


321 


as  the  object  before  it :  this  is  owing  to  the  fact  that  the 
reflected  rays  come  from  the  glass  at  the  same  angle  that 
the  incident  rays  strike  upon  it  (§  215).  This  may  be  shown 
from  Fig.  261.  Suppose  m  m'  is  a  looking-glass,  and  the 
arrow,  A  B,  is  placed  before  it.  Rays  of  light  pass  from  it 
at  all  points  to  the  glass.  We 
will  consider  the  path  of  only 
two  of  these  rays  at  each  end  of 
the  arrow.  The  ray  A  g  will  be 
reflected  to  the  eye  at  the  same 
angle  in  the  ray  g  o,  and  the  ray 
A/ will  be  reflected  in  the  ray 
/E.  And  the  reflected  rays 
will  have  the  same  rate  of  diver- 
gence as  the  incident  rays.  The 
same  can  be  shown  in  regard  to 
rays  from  B  or  any  other  point 
on  the  arrow.  Now,  if  the  lines 
o  g  and  E/be  prolonged,  they  will  meet  at  the  point  a, 
which  lies  at  the  same  distance  behind  the  mirror  as  A  is 
before  it.  The  same  thing  can  be  shown  of  the  rays  from 
B  or  any  other  point.  Therefore  the  image  of  the  arrow 
will  appear  to  the  eye  to  have  the  same  relative  position 
behind  the  glass  that  the  arrow  itself  has  before  it. 

218.  The  Kaleidoscope. — We  have  already  noticed  the 
multiplication  of  the  images  of  objects  by  using  two  or 
more  mirrors.  In  the  scientific  toy  called  a  kaleidoscope, 
by  a  particular  arrangement  of  mirrors  the  images  are 
multiplied,  and  by  changes  in  the  position  of  the  objects 
the  relative  positions  of  the  images  are  infinitely  varied. 
Fig.  262  will  serve  to  explain  the  operation  of  the  instru- 
ment. Let  A  B  and  B  C  be  two  plane  mirrors  placed 
at  right  angles  to  each  other,  and  a  an  object  before 
them.  Let  I  be  the  position  of  the  eye  looking  at  the 


522 


NATURAL   PHILOSOPHY. 


Fig.  262. 


mirrors.  The  rays  a 
a  g  will  be  reflected  to  I 
as  represented,  and  the  eye 
will  see  two  images,  which 
appear  to  be  at  b  and  E. 
But  the  ray  a  K  will  be  re- 
flected to  c,  and  then  to  I, 
so  that  a  third  image  will 
be  seen  at  d.  In  this  case 
but  a  single  second  reflec- 
tion, or  reflection  of  an  im- 
age is  formed;  but  by  placing  the  mirrors  at  an  angle  of 
60°, 45°, and  30° the  images  maybe  increased  in  number  to 
five,  seven,  and  nine,  having  a  circular  arrangement.  In  the 
kaleidoscope  two  mirrors  are  placed  in  a  tube  at  an  angle 
of  30°;  and  variously  colored  pieces  of  glass  in  the  farther 
end  of  the  instrument,  changing  their  relative  position  with 
every  movement,  give  an  endless  variety  of  images  symmet- 
rically arranged. 

219.  Effect  of  Curved  Mirrors. — Curved  mirrors  are  chief- 
ly of  two  kinds — concave,  or  hollowed  out,  and  convex,  or 
bulging  out.  The  manner  in  which  light  is  reflected  from 
a  concave  mirror  may  be  illustrated  by  Fig.  263.  If  paral- 
lel rays  strike  upon  the  mirror,  they 
will  be  made  to  converge,  or  come 
together,  at  the  point  a,  called  the 

focus.  If,  however,  the  source  of  light 

be  placed  at  the  focus  itself,  the  rays    

striking  the  mirror,  as  shown,  will 

diverge,  and  the  reflected  rays  will 

be  parallel.     If  the  light  or  object 

be  nearer  to  the  mirror  than  the  focus,  and  the  rays,  of 

course,  diverge  more,  then  the  eifect  of  the  mirror  will 

be  to  lessen  the  divergency  when  the  rays  are  reflected. 


LIGHT. 


323 


Fig.  2C4. 


You  see  that  the  tendency  is  to  make  the  rays  converge. 
And  hence  concave  reflectors  are  much  used  when  it  is 
desired  to  throw  a  great  amount  of  light  in  one  direc- 
tion. The  effect  of  the  concave  mirror  upon  the  apparent 
size  and  position  of  objects  placed 
before  it  varies  with  the  relation  of 
their  position  to  the  focus. 

The  action  of  a  convex  mirror 
upon  light  is  the  opposite  of  that  of 
the  concave.  Its  tendency  is  to  make 
the  rays  diverge.  Thus,  if  parallel 
rays  strike  upon  a  convex  mirror 
(Fig.  264),  they  diverge,  as  if  they 
carne  from  a  focus  behind  the  mirror 
at  £,  indicated  by  the  dotted  lines. 

220.  Refraction  of  Light. — The  substance  through  which 
light  (or  any  other  agent)  moves  in  passing  from  one  point 
to  another  is  called  a  medium ;  thus  air,  glass,  water,  etc., 
are  media.  Now,  light  moves  in  straight  lines  so  long  as 
it  passes  through  a  medium  of  uniform  density;  but  if  it 
pass  from  a  denser  into  a  rarer,  or  from  a  rarer  into  a 

denser  medium,  it  is  bent  from 
its  course.  This  bending,  or 
refraction,  as  it  is  called,  takes 
place  only  when  the  ray  of 
light  enters  or  leaves  the  me- 
dium in  an 'oblique  direction. 
Thus,  if  a  ray  of  light  pass- 
ing from  air  into  the  denser 
medium  water  strike  the  lat- 
ter perpendicularly  to  the  sur- 
face,£>'^>,  in  Fig.  265,  no  refrac- 
tion or  alteration  in  its  course 
Fig.  265.  takes  place ;  but  if  it  strike  the 


324  NATUKAL   PHILOSOPHY. 

water  obliquely,  as  indicated  by  the  line  I  n,  it  is  bent  in 
the  direction  n  s.  When  light  passes  into  a  denser  medi- 
um, it  is  refracted  toicards  the  perpendicular  let  fall  upon 
the  point  of  contact,  the  angle  r  being  smaller  than  the 
angle  l\  when,  however,  light  passes  into  a  rarer  medium, 
the  reverse  takes  place :  it  is  bent  from  the  perpendicular 
let  fall  upon  the  point  at  which  the  emergence  occurs. 

It  is  owing  to  this  refraction  of  light  that  a  straight  stick 
partly  immersed  in  water  appears  to  the  eye  to  be  bent 
just  at  the  surface  of  the  water. 

Tig.  266  also  furnishes  another 
illustration.  Let  the  vessel  shown 
be  empty,  and  place  a  coin  at  m. 
Then  place  your  eye  at  such  a 
point,  a,  that  the  rim  of  the  ves- 
sel just  conceals  the  coin  from 
view.  If  you  then  pour  in  water 
up  to  a  certain  level,  say  v  v',  the 
coin  will  come  in  view.  This  is 
because  light  reflected  from  the 

coin  to  i  i  is  bent  in  the  direction  i  a,  and  therefore  appears  to  the  eye  to 
be  at  n.  In  this  case  the  refraction  is  from  the  perpendicular. 

We  have  taken  as  examples  water  and  air,  but  more  or  less  refraction 
occurs  whenever  light  passes  from  one  transparent  body  into  another. 
Different  substances  differ  much  in  their  power  of  thus  bending  light;  thus, 
glass  possesses  a  higher  refractive  power  than  water,  rock-salt  than  glass, 
and  the  diamond  most  of  all.  It  is  to  this  high  light-bending  power  that 
the  diamond  owes  in  part  its  sparkling  -brilliancy.  Bearing  in  mind  that 
light  is  vibrations  of  ether,  we  may  regard  the  refraction  by  a  denser  medi- 
um as  a  retardation  of  these  vibrations,  and  the  refraction  which  takes 
place  when  light  enters  a  rarer  medium  as  an  acceleration  of  the  same. 

221.  Dawn  and  Twilight. — The  light  of  the  sun,  in  pass- 
ing from  space  into  our  atmosphere,  is  refracted.  Other- 
wise we  should  have  no  daylight  preceding  the  rise  of  the 
sun,  nor  twilight  after  its  setting ;  but  light  would  burst 
upon  the  darkness  of  night  as  soon  as  the  sun  appeared 


LIGHT.  325 

above  the  horizon,  and  darkness  would  suddenly  succeed  »> 
to  the  light  of  day  at  sunset.  As  it  is,  in  the  morning  the 
light  bends  towards  us  in  passing  through  the  atmosphere 
long  before  we  see  the  sun,  and  after  the  sun  has  disap- 
peared from  view  at  evening  its  light  bends  towards  us  in 
the  same  manner.  And,  further,  we  really  see  the  sun  in 
the  morning  before  it  gets  above  the  horizon,  and  in  the 
evening  after  it  has  gone  below  it.  This  may  be  made  clear 
by  Fig.  267.  Let  the  central  ball  represent  the  earth. 
Now,  since  the  atmosphere  is 
most  dense  near  the  earth, 
and  is  rarer  as  you  go  out- 
ward from  the  earth,  it  is  rep- 
resented in  the  figure  as  hav- 
ing different  layers  in  order 
that  the  operation  of  the  re- 
fraction may  be  more  easily  Fig.  26r. 
explained.  The  light  issuing  from  the  sun,  S,  below  the  ho- 
rizon, on  reaching  the  first  layer  of  air,  instead  of  passing  on 
straight  to  a,  as  indicated  by  the  dotted  line,  bends  tow- 
ards the  earth.  Then  in  entering  the  second  layer,  instead 
of  passing  on  to  5,  it  is  bent  or  refracted  still  more ;  and  so 
on  through  all  the  layers,  being  refracted  in  each  more 
than  in  the  previous  one.  The  result  is,  that  the  sun, 
though  really  below  the  horizon,  appears  to  be  above  it. 
The  path  of  light  from  the  sun,  as  it  passes  through  the  air, 
is  a  curved  line.  This  is  because  the  air,  instead  of  being 
of  uniform  density,  lessens  in  density  as  we  go  from  the 
earth.  If  it  were  of  uniform  density,  the  light  would  be 
refracted  in  straight  lines,  as  in  the  experiments  in  §  220. 

222.  Mirages. — Sometimes  inequalities  occur  in  the  den- 
sity of  the  lower  portions  of  the  atmosphere,  causing,  of 
course,  unequal  refraction,  and  producing  strange  appear- 
ances, termed  mirages.  For  example,  at  Ramsgate,  on  the 

02 


326 


NATURAL  PHILOSOPHY. 


coast  of  England,  on  a  certain  occasion,  a  ship  was  seen 
at  such  a  distance  that  only  her  topsails  were  visible ;  and 
above  in  the  air  there  were  two  complete  images  of  the 
ship,  the  uppermost  being  erect  and  the  under  one  invert- 
ed (Fig.  268).  Captain  Scoresby,  in  a  voyage  to  Green- 


Fig.  26S. 

land,  saw  an  inverted  image  of  a  ship  so  well  defined  that 
he  decided  that  it  was  the  image  of  his  father's  ship,  the 
Fame,  which  was  afterwards  verified.  The  ships  were  at 
that  time  separated  a  distance  of  thirty  miles. 

An  incident  in  the  early  history  of  the  author's  place  of 
residence  may  be  cited  as  an  example  of  mirage.  A  ship 
left  New  Haven  for  England  freighted  with  a  valuable 
cargo,  and  having  on  board  a  large  number  of  the  best  cit- 
izens of  the  colony.  Some  time  after  there  was  immense 
excitement  in  New  Haven,  because  the  inhabitants  saw, 
with  great  distinctness,  what  they  supposed  to  be  this  ves- 
sel, at  only  a  little  distance,  apparently  sailing  against  the 


LIGHT. 


327 


wind.  But  it  soon  disappeared  from  view,  little  by  little, 
until  the  whole  was  gone.  The  ship  itself  was  never  heard 
from,  and  it  was  supposed  at  the  time  that  this  appearance 
was  a  manifestation  of  Providence  for  the  purpose  of  in- 
forming the  colonists  what  had  become  of  their  friends. 
But  that  which  was  seen  was  undoubtedly  the  reflected 
image  of  this  or  some  other  ship.  Such  appearances  as 
these  have  given  rise  to  the  stories  sometimes  told  of 
phantom  ships.  Mirages  are  very  common  in  the  exten- 
sive deserts  in  hot  climates,  exhibiting  to  the  eye  of  the 
traveller  various  deceptive  appearances,  as  islands,  lakes, 
etc.  In  Bonaparte's  campaign  in  Egypt  such  an  appear- 
ance on  one  occasion  caused  whole  battalions  of  thirsty  sol- 
diers to  rush  forward,  supposing  at  the  moment  that  a  plen- 
tiful supply  of  water  was  at  hand. 

223.  Visual  Angle. — In  order  that  you  may  understand 
the  operation  of  lenses  in  relation  to  vision,  we  must  first 
explain  what  is  meant  by  the  visual  angle.  In  Fig.  269 


Fig.  269. 

are  represented  arrows  of  the  same  size  at  different  dis- 
tances from  the  eye.  From  the  ends  of  each  of  the  ar- 
rows are  drawn  lines  to  the  eye.  The  angle  which  these, 
lines  make  in  each  case  as  they  meet  at  the  eye  is  .termed 
the  visual  angle.  Now,  the  apparent  size  of  an  object  de- 
pends upon  the  size  of  this  angle.  The  degrees  of  the  an- 
gles are  marked  upon  the  arrows.  Thus  the  visual  angle 
of  the  nearest  arrow  is  120  degrees,  and  that  of  the  second 
is  60,  only  half  as  large.  The  first  arrow  therefore  appears 
twice  as  large  as  the  second.  For  the  same  reason,  it  ap- 
pears four  times  as  large  as  the  third,  eight  times  as  large 


328 


NATUKAL  PHILOSOPHY. 


Fig. 2TO. 


as  the  fourth,  and  twelve 
times  as  large  as  the  fifth. 
The  same  thing  is  illustrated 
in  another  way  in  Fig.  270. 
Here  the  arrows  e  /,  g  h, 
and  i  k  appear  to  the  eye 
as  large  as  A  B,  because 
they  have  the  same  visual  angle,  and  for  this  reason  make 
an  image  of  the  same  size  in  the  eye,  as  indicated  in  the 
figure.  It  is  hardly  necessary  to  say  that  what  is  true  of 
objects  as  a  whole  is  also  true  of  any  part  of  them.  Each 
part,  however  small,  has  its  visual  angle,  and  this  governs 
its  apparent  size. 

224.  Lenses. — Lenses  are  transparent  bodies  having 
curved  surfaces  and  possessing  the  power  of  increasing  or 
diminishing  the  convergence  or  divergence  of  the  rays  of 
light  which  pass  through  them.  Lenses  are  usually  made 
of  glass,  but  they  can  be  made  of  any  transparent  solid,  as 
quartz,  ice,  etc.  There  are  six  different  forms  of  lenses,  as 
represented  in  Fig.  271.  No.  1,  being  convex  on  both  sides, 


Fig.  271. 

is  called  a  double  convex  lens ;  2  is  a  plano-convex  lens ;  3 
is  concavo-convex;  4  double  concave;  5  plano-concave; 
and  6  concavo-convex.  3  and  6  are  also  called  meniscuses. 
The  lenses  in  most  common  use  are  the  double  convex  (1) 
and  the  double  concave  (4).  The  explanation  of  the  manner 


LIGHT.  329 

in  which  these  act  upon  light  will  sufficiently  illustrate  the 
operation  of  the  others.  They  act  by  refraction,  the  con- 
vex collecting  the  rays,  or  bringing  them  nearer  together, 
and  the  concave  spreading  them  farther  apart.  You  can 
at  once  see  that  a  convex  lens,  by  causing  the  rays  com- 
ing from  an  object  to  converge  more,  increases  the  visual 
angle,  and  therefore  makes  the  object  to  appear  larger 
than  it  otherwise  would.  This  effect  is  illustrated  by 
Fig.  272.  The  rays  of  light  reflected  from  the  arrow 
are  made  by  the  lens  to  converge  so 
as  to  meet  at  a,  instead  of  at  b ;  that  is, 
by  passing  through  the  lens  they  have 
a  larger  visual  angle,  and  therefore  the 
object  is  magnified.  The  distance  be- 
tween  c  and  d  shows  the  size  which 
the  arrow  would  appear  to  have  to  the 
eye  placed  at  a.  In  a  similar  manner 
the  double  concave  lens  causes  the  rays 
to  diverge,  and,  consequently,  objects  d/ 

S    '.     .  Vi  ii  Fig.  272. 

seen  through  it  appear  to  be  smaller. 

The  magnifying  power  of  any  lens  depends  on  the  degree 
of  its  convexity,  or  the  bulging  of  its  surfaces ;  for  the  less 
it  bulges,  the  nearer  it  is  to  a  plane  surface ;  and  the  more 
it  is  curved  outward,  the  more  obliquely  will  the  rays  fall 
upon  its  surface,  and  the  greater  therefore  will  be  the  re- 
fraction which  brings  them  to  a  focus.  A  minute  sphere 
forms  a  very  powerful  magnifying-glass. 

225.  Microscopes  and  Telescopes. — What  has  been  said 
of  the  action  of  the  convex  lens  upon  the  visual  angle  ex- 
plains also  the  operation  of  the  microscope.  Microscopes 
may  be  simple  or  compound.  Simple  microscopes  contain 
only  one  lens,  the  action  of  which  has  been  described.  Com- 
pound microscopes  used  to  magnify  and  render  visible  ex- 
ceedingly minute  objects  contain  combinations  of  two  or 


330 


NATURAL  PHILOSOPHY. 


Fig. 273. 


more  lenses.    The  operation  of  a  simple  form  of  this  instru- 
ment is  shown  in  Fig.  273.     Rays  from  the  object,  E  F, 

passing  through  the  first  lens, 
or  object-glass,  as  it  is  called, 
form  a  magnified  inverted 
image,  G  H,  which  is  still 
more  magnified  by  the  eye- 
glass, C  D. 

In  the  telescope,  convex 
lenses  are  also  employed,  but  the  arrangement  is  different, 
since  the  objects  to  be  magnified  are  very  distant.  Fig.  274 
shows  the  path 
of  the  rays  of 
light  through 
the  two  convex 
lenses,  forming 
a  simple  astronomical  telescope.  Light  proceeding  from 
the  object,  A,  passes  through  the  object-glass,  L,  to  form 
an  image  at  b  a,  which  image  is  seen  through  the  eye-piece, 
D,  the  rays  entering  the  eye  at  E.  Of  course  the  image  is 
seen  in  an  inverted  position,  but  this  is  of  no  consequence 
in  viewing  heavenly  bodies,  which  are  spherical. 

226.  Magic  Lantern. — This  is  an  instrument  by  which 
greatly  enlarged  transparent  pictures  are  made  visible  by 
being  thrown  upon  a  screen.  The  pictures  are  either  painted 
with  transparent  colors  upon  glass,  or  directly  photographed 
upon  the  same  material.  The  apparatus,  in  its  simplest  form, 
consists  of  a  metallic  lantern,  A  A,  Fig.  275,  having  a  concave 
reflector,  p  q,  and  two  convex  lenses,  m  and  n.  At  c  d  is  a 
space  between  the  lenses  into  which  the  pictures  are  intro- 
duced. Lisa  strong  light,  which  is  in  the  focus  both  of  the 
mirror  and  the  lens  m.  The  picture  is  therefore  illuminated 
strongly  by  the  rays,  reflected  from  the  mirror  and  passed 
through  the  lens.  The  lens  n,  which  is  movable,  is  so  ad- 


LIGHT. 


331 


Pig.  275. 

justed  as  to  throw  a  highly  magnified  image  of  the  picture 
upon  the  screen.  Since  the  image  is  an  inverted  one,  the 
pictures  must  be  inserted  upside  down.  In  a  more  modern 
form  of  this  instrument,  to  which  the  name  stereopticon  has 
been  given,  an  oxyhydrogen  light  is  substituted  for  the  com- 
mon lamp.  The  solar  microscope  is  essentially  similar,  the 
sun  serving  as  the  illuminator  and  three  lenses  being  used. 
227.  Camera  Obscura. — This  instrument  differs  from  the 
magic  lantern  in  giving  us  diminished  images  of  objects. 
An  instrument  of  this  kind  can  be  arranged  extemporane- 
ously by  any  one.  Thus,  if  into  a  darkened  chamber  light 
be  admitted  through  a  small  opening,  inverted  images  of 
any  objects  in  front  of  the  opening  will  be  formed  upon  a 
white  screen^in  the  opposite  part  of  the  chamber.  Such  an 
arrangement  is  represented  in  Fig.  276.  The  images  in 


332 


NATUKAL  PHILOSOPHY. 


such  a  case,  however,  are  faint,  because  the  opening  must 
necessarily  be  small,  and  therefore  but  few  rays,  compara- 
tively, come  from  the  objects.  By  making  the  opening 
larger,  and  gathering  the  rays  that  enter  it  with  a  double 
convex  lens,  we  can  obtain  well-defined  and  bright  images 
of  objects. 

Fig.  277  represents  another  form  of  this  apparatus,  call- 


A< 


2T7. 


ed  a  camera  lucida,  used  for  sketch- 

^W^B  *n°    sm§'^e  objects   and  landscapes. 

I — "JljOLleL.  In  this  the  rays  of  light  reflected 
from  objects  strike  upon  a  mirror, 
A  B,  and  are  reflected  through  a  con- 
vex lens,  C  D,  upon  white  paper  on 
the  bottom,  E  F,  of  the  box,  where 
the  outlines  of  the  images  are  traced 
by  the  sketcher.  The  light  can  en- 
ter only  at  the  opening  above,  for  on 
the  side  of  the  box  which  is  open 
there  hangs  down  a  curtain  on  the 
back  of  the  artist  while  he  sketches. 

228.  The  Eye. — The  eye  is  essentially  a  camera  obscura. 
It  is  a  dark  chamber  in  which  images  are  formed  upon  a 
screen  in  its  back  part,  and  the  light  which  cqmes  from  ob- 
jects.is  admitted  through  an  opening  in  front,  where  there 
is  a  double  convex  lens.  The  sectional  drawing  of  the  eye 
(Fig.  278)  will  enable  you  to  understand  the  manner  in  which 
the  images  are  formed.  At  a  is  the  thick,  strong  white  coat 
called  the  sclerotic  coat,  from  a  Greek  word  meaning  hard. 
This,  which  is  commonly  called  the  white  of  the  eye,  gives 
to  the  eyeball  its  firmness.  Into  this  is  fastened  in  front, 
like  a  crystal  in  a  watch-case,  e,  the  cornea.  The  sclerotic 
and  cornea  make  together  one  coat  of  the  eye — the  outer 
one.  The  cornea  is  the  clear,  transparent  window  of  the 
eye  through  which  the  light  enters.  Next  to  the  sclerotic 


LIGHT.  333 


coat  comes  the  choroid  coat,  b,  which  is  dark,  to  prevent 
too  much  reflection  back  and  forth  in  the  eye.  The  very 
thin  membrane,  c,  called  the  retina,  serves  as  a  screen 
on  which  the  images  are  formed.  This  is  composed  chief- 
ly of  the  fine  fibres  of  the  optic  nerve,  d.  To  return  to 
the  front  of  the  eye  where  the  light  enters — behind  the 
cornea  is  the  iris,  g  g,  on  which  depends  the  color  of  the 
eye;  this  is  immersed  in  a  watery  fluid,/,  called  the  aque- 
ous humor.  The  light  passing  through  the  cornea  and  the 
aqueous  humor,  reaches  the  crystalline  lens,  h,  which  is  a 
double  convex  lens.  Passing  through  this  and  through 
a  jelly-like  substance  called  the  vitreous  humor,  which 
fills  all  that  large  space,  i,  it  strikes  upon  the  retina,  c, 
where  it  forms  the  images  of  the  objects  from  which  it 
came.  These  impressions  are  then  transmitted  to  the  brain 
by  means  of  the  optic  nerve,  d. 

You  now  see  how  the  eye  resembles  a  camera  obscura : 
like  the  latter,  it  has  the  dark  chamber  with  its  screen;  the 
opening  through  the  iris,  the  pupil,  for  the  admission  of  the 
light;  and  just  behind  this  opening  the  lens  for  gathering 
or  concentrating  the  light  before  it  falls  upon  the  retina. 


334 


KATURAL  PHILOSOPHY. 


The  refraction  of  the  light  is  not,  however,  wholly  due  to 
this  lens.  The  projecting  cornea,  with  its  contained  aque- 
ous humor,  refracts  it  considerably,  for  it  forms  a  convex 
lens. 

229.  Distinct  Vision. — In  order  that  vision  may  be  per- 
fectly distinct,  it  is  necessary  that  the  rays  from  each  point 
of  the  object  seen  should,  on  converging,  meet  together,  or 
be  brought  to  a  focus  on  the  screen  of  the  eye,  the  retina. 
Thus,  in  Fig.  279,  the  rays  from  a,  one  end  of  the  arrow, 

meet  on  the  retina  at  Z>, 
and  those  from  c,  the  oth- 
er end,  are  brought  to  a 
focus  at  d.  The  muscles 
of  the  eye  have  the  pow- 
er of  adjusting  the  eye  to  objects  at  different  distances, 
so  as  in  most  cases  to  bring  the  rays  together  exact- 
ly at  the*  retina.  They  fail  to  do  it  with  objects  that 
are  very  near.  You  can  perceive  that  this  is  the  case 
if  you  bring  any  object,  as  your  finger,  nearer  and  nearer 
to  the  eye.  You  will  find  that  at  length  you  cannot 
see  it  distinctly.  The  reason  is,  that  the  rays  from  it 
diverge  so  much  that  the  cornea  and  lens  cannot  make 
them  converge  enough  to  meet  at  the  retina.  This  diver- 
gence of  rays  at  different  distances  is  illustrated  in  Fig. 
280.  When  you  look  at  a  very  mi- 
nute object,  the  nearer  you  bring  it 
to  the  eye,  the  better  you  can  see  it, 
up  to  a  certain  point.  There  the 
rays  are  so  divergent,  as  is  evident 
in  the  figure,  that  the  lenses  of  the 
eye  cannot  make  them  converge  suf- 
ficiently for  distinct  vision.  The  microscope  assists  the  eye 
by  causing  these  divergent  rays  to  come  nearer  together 
before  they  enter  the  window  of  the  eye,  the  cornea. 


Fig.  280. 


LIGHT.  335 

Near-sighted  and  Far-sighted  Eyes. — Some  persons  have  eyes  so  shaped 
that  they  cannot  fully  adjust  them  to  objects  at  different  distances.  Thus, 
the  near-sighted  can  see  with  distinctness  only  those  objects  comparative- 
ly close  to  the  eye.  This  is  because  the  rays  converge  too  much,  and 
are  brought  to  a  focus  before 
they  fall  on  the  retina,  as  repre- 
sented in  Fig.  281.  The  images, 
therefore,  of  distant  objects  are  in- 
distinct. If  the  retina  could  in 
any  way  be  brought  forward  a 
little,  the  difficulty  would  be  ob- 
viated. But  since  this  cannot  be  done,  concave  glasses  are  resorted  to, 
which  counteract  the  effect  of  the  too  great  refractive  power  of  the  eye.  In 

the  far-sighted  the  difficulty  is 
of  an  opposite  character.    The 
refractive  power  is  so   feeble 
that   when    near   objects    are 
viewed,  the  rays  are  not  brought 
to  a  focus  soon  enough,  and 
fall  at  a  point  behind  the  reti- 
na, as  shown  in  Fig.  282.    Convex  glasses  are  used  in  this  case,  making  the 
divergent  rays  of  near  objects  less  divergent  before  they  enter  the  cornea. 

230.  Images  in  the  Eye  Inverted. — The  images  formed  on 
the  retina  are  inverted.  This  can  be  proved  by  taking  the 
eye  of  an  ox  and  carefully  paring  oiF  the  back  of  it,  leaving 
little  else  than  the  retina  itself.  Holding  now  a  candle  be- 
fore the  eye,  its  image  may  be  seen  inverted  upon  the  rear 
part.  The  question  arises  why  is  it  that  we  see  objects 
erect  when  their  images  on  the  retina  are  inverted  ?  On 
this  point  we  will  quote  from  Hooker's  Human  Physiology  : 

"It  has  been  supposed  by  some  that  we  really  see  everything  reversed, 
and  that  our  experience  with  the  sense  of  touch,  in  connection  with  that  of 
vision,  sets  us  right  in  this  particular.  And  this,  it  is  supposed,  is  the  more 
readily  done  from  the  fact  that  our  own  limbs  Mid  bodies  are  reversed  as 
pictured  on  the  retina,  as  well  as  objects  that  are  around  us,  so  that  every- 
thing is  relatively  right  in  position.  But  if  this  be  the  true  explanation, 
those  who  have  their  sight  restored  after  having  been  blind  from  birth 


336  NATURAL   PHILOSOPHY. 

should  at  first  see  everything  wrong  side  up,  and  should  be  conscious  of 
rectifying  the  error  by  looking  at  their  own  limbs  and  bodies.  But  this  is 
not  the  case.  The  above  explanation  of  erect  vision,  and  other  explana- 
tions of  a  similar  character,  are  based  upon  a  wrong  idea  of  the  office  which 
the  nerve  performs  in  the  process  of  vision.  It  is  not  the  image  formed 
upon  the  retina  which  is  transmitted  to  the  brain,  but  an  impression  pro- 
duced by  that  image.  The  mind  does  not  look  in  upon  the  eye  and  see 
the  image,  but  it  receives  an  impression  from  it  through  the  nerve ;  and 
this  impression  is  so  managed  that  the  mind  gets  the  right  idea  of  the  rela- 
tive position  of  objects.  Of  the  way  in  which  this  is  done  we  know  as  little 
as  we  know  of  the  nature  of  the  impression  itself." 

231.  Single  Vision. — Whenever  we  see  any  object  with 
both  eyes,  an  image  is  formed  in  each  eye,  and  impressions 
pass  from  both  eyes  by  the  optic  nerve  to  the  brain.     And 
yet  these  two  impressions  produce  no  double  vision  so  long 
as  the  two  eyes  correspond  with  each  other  in  situation. 
This  is  because  the  image  in  one  eye  occupies  the  same 
place  on  the  retina  as  that  in  the  other.     The  correspond- 
ence is  ordinarily  perfect,  the  two  eyes  turning  always  to- 
gether in  the  same  way,  upward,  downward,  or  laterally, 
without  the  least  variation.    You  can  observe  the  effect  of 
a  want  of  this  correspondence  by  pressing  one  of  the  eyes 
with  the  finger  while  the  other  is  left  free  to  move  in  obe- 
dience to  the  muscles.    When  this  is  done,  every  object  ap- 
pears double,  because  its  image  occupies  in  one  eye  a  dif- 
ferent part  of  the  retina  from  that  which  it  does  in  the  oth- 
er, and  hence  two  different  impressions  are  carried  to  the 
brain.     The  same  thing  occurs  in  squinting,  in  which  the 
action  of  the  muscles  of  the  two  eyes  does  not  agree.     Or- 
dinarily in  squinting  there  is  no  double  vision,  because  the 
mind  has  the  habit  of  disregarding  the   impressions  re- 
ceived from  the  defective  eye ;  but  when  squinting  occurs 
suddenly  from  disease,  double  vision  ensues,  for  it  takes  a 
little  time  to  form  the  habit  referred  to. 

232.  Stereoscope.  —  The  images  of  objects  in  the  two 


LIGHT.  337 

eyes,  though  always  similar,  are  generally  not  precisely 
alike.  They  are  so  only  when  the  object  presents  a  simple 
plane  surface,  as  in  the  case  of  pictures.  When  the  object 
presents  two  or  more  surfaces  to  the  sight,  the  images  are 
more  or  less  unlike.  This  can  be  illustrated  in  a  very  sim- 
ple way.  Hold  a  book  up  straight  before  your  eyes  with 
its  back  towards  you.  You  see  the  back  and  both  sides. 
Now,  if  you  shut  your  right  eye,  you  will  see  with  the  left 
the  back  of  the  book  and  the  left  side;  that  is,  these  two 
parts  of  the  book  are  imaged  on  the  retina  of  the  left  eye. 
By  shutting  the  left  eye  it  will  appear  that  the  image  in 
the  right  is  different,  for  you  see  now  with  the  back  the 
right  side  of  the  book.  These  simple  phenomena  underlie 
the  principles  on  which  the  instrument  known  as  a  stereo- 
scope is  constructed.  In  the  right  side  of  this  instrument 
is  placed  the  picture  of  the  object  as  the  object  itself  would 
appear  to  the  right  eye,  and  in  the  left  side  is  placed  the 
picture  as  it  would  appear  to  the  left  eye.  Thus,  if  a  book 
in  the  position  alluded  to  above  were  the  object,  in  the 
right  picture  there  should  be  represented  the  back  together 
with  the  right  side  of  the  cover,  and  in  the  left  the  back 
with  the  left  side  of  the  cover.  The  two  impressions,  car- 
ried to  the  brain  by  the  optic  nerves,  give  together  the  im- 
pression of  a  solid  book.  The  same  principles  apply  to  the 
representation  of  all  solids  in  the  stereoscope. 

233.  Thaumatrope. — Each  impression  made  upon  the  ret- 
ina by  light  lasts  about  one-sixth  part  of  a  second ;  accord- 
ing to  some,  it  is  only  one  eighth  or  one  eleventh  of  a  sec- 
ond. No  distinct  impressions  can  be  made,  therefore,  upon 
the  retina  unless  they  succeed  each  other  with  less  rapidity 
than  this.  If,  for  example,  in  the  revolution  of  a  wheel, 
eight  or  more  spokes  pass  by  one  point  in  a  second,  they 
cannot  be  seen  as  distinct  spokes,  but  appear  confused, 
producing  one  continuous  impression.  If  a  light  be  re- 


338 


NATURAL   PHILOSOPHY. 


volved  so  as  to  describe  a  circle  in  an  eighth  part  of  a  sec- 
ond, it  will  appear  to  the  eye  as  one  unbroken  circle  of 
light.  It  is  this  persistence  of  impressions  on  the  retina 
that  makes  small  objects  seen  from  a  swiftly  moving  rail- 
road-car appear  to  run  in  long  lines  along  with  us.  This 
principle  is  made  use  of  in  the  construction  of  a  toy  called 
the  thaumatrope.  Pictures  are  drawn  on  each  side  of  a 
circular  card,  and  when  it  is  whirled  around  very  rapidly 
by  means  of  two  strings  fastened  to  it,  the  two  pictures 

appear  as  one.  Thus, 
in  Fig.  283  are  repre- 
sented the  two  sides 
of  such  a  card,  one 
side  having  the  pict- 
Fjg-283-  ure  of  a  dog,  and  the 

other  that  of  a  monkey.  When  made  to  revolve  rap- 
idly, the  monkey  will  appear  to  be  sitting  on  the  back  of 
the  dog.  Several  other  pieces  of  apparatus  have  been  de- 
vised to  illustrate  the  persistence  of  vision,  such  as  the 
zoetrope,  phenakistoscope,  kaleidophone,  etc.,  the  popular 
expression  "an  optical  illusion"  being  commonly  employed 
to  explain  this  whole  class  of  scientific  toys. 

234.  Decomposition  of  White  Light. — The  manner  in 
which  light  is  refracted  in  passing  from  one  medium  to 
another  of  greater  or  less  density  has  been  explained  in 
§  220,  but  we  must  again  return  to  the  subject.  Let  A  13, 
Fig.  284,  represent  the  section  of 
a  piece  of  glass  having  parallel 
surfaces.  When  a  ray  of  light 
passes  through  at  right  angles 
to  its  surface,  as  c  d,  it  suffers 
no  refraction  ;  when,  however,  it 
strikes  the  glass  obliquely,  as  ef, 
it  is  refracted  both  at  /,  where  it 


LIGHT. 


339 


enters  the  glass,  and  at  #,  where  it  leaves  it ;  its  course  on 
emerging,  y  h,  being  parallel  to  its  original  path,  ef. 

A  piece  of  glass  the  section  of  which  forms  a  triangle  is 
called  a  prism.  Fig.  285 
represents  such  a  prism. 
Somewhat  similar  pieces 
of  glass  were  former- 
ly used  for  decorating 
chandeliers.  When  a  ray  of  light  passes  through  a  prism, 
it  is  twice  refracted,  just  as  in  passing  through  a  plate 
of  glass  with  parallel  surfaces,  but,  instead  of  issuing  in  a 

path  parallel  to  that  by  which 
it  entered,  it  is  bent  entirely 
out  of  its  course ;  thus,  the  ray 
of  light,  d  e,  passing  through 


the  prism,  ABC,  Fig.  286, 
Fig.  286.  takes  the  course  d  e  f  g. 

The  explanation  of  this  depends  upon  the  fact  already 
mentioned  (§  220),  that  a  ray  of  light  passing  from  a  rare 
into  a  denser  medium  is  bent  towards  the  perpendicular, 
and  from  a  dense  into  a  rarer  medium  it  is  bent/rora  the 
•perpendicular  let  fall  upon  the  surface.  This  will  be  easily 
•understood  by  reference  to  Fig.  284,  where  the  ray  ef\$ 
bent  towards  the  perpendicular  if,  and  from  the  perpen- 
dicular kg  as  it  respectively  enters  and  leaves  the  dense 
medium.  In  like  manner  the  ray  d  e,  entering  the  prism, 
ABC,  Fig.  286,  is  bent  towards  the  perpendicular,  k  e,  and 
on  emerging  at /"is  bent  from  the  perpendicular,  if,  thus 
pursuing  a  very  crooked  path. 

When  we  make  the  experiment  of  passing  a  ray  of  light 
from  a  small  orifice  through  a  glass  prism,  we  observe,  how- 
ever, something  more  than  the  mere  bending  of  the  ray :  we 
obtain  a  beautiful  image,  having  all  the  colors  of  the  rain- 
bow, called  a  spectrum. 


340 


NATURAL  PHILOSOPHY. 


Fig.  287. 

Fig.  287  illustrates  this  dispersion  of  white  light,  as  the 
phenomenon  is  called.  Let  the  beam  of  sunlight,  D  E,  pass 
through  a  small  opening  in  a  shutter  into  a  dark  room. 
The  rays  will  pursue  a  straight  course;  and  if  a  screen  be 
placed  at  F,  they  will  form  a  spot  of  white  light.  But  if  a 
glass  prism,  A  B  C,  be  held  in  the  position  represented,  the 
rays  will  be  refracted,  and  when  received  upon  the  screen, 
M  N",  the  light  will  be  decomposed  into  the  various  colored 
rays  of  which  it  is  constituted.  The  whole  beam  is  refract- 
ed for  reasons  just  given,  and  the  separation  or  dispersion 
takes  place  because  the  rays  of  different  colors  are  unequal- 
ly refracted.  If  they  were  equally  refracted,  the  light  upon 
the  screen  would  be  white,  as  before  the  dispersion.  The 
violet  rays  are  most  refracted,  the  indigo  next,  the  blue 
next,  etc.,  the  red  being  the  least  refracted  of  all. 

The  light  of  the  sun  passed  through  a  prism  yields  a  so- 
lar spectrum,  familiarly  seen  in  the  rainbow.  When  lights 
from  other  sources  are  similarly  examined,  different  spectra 
are  obtained  characteristic  of  the  sources.  This  fact  gives 
rise  to  the  science  of  spectrum  analysis,  which  will  be  fully 
explained  in  the  twenty-second  chapter  of  Part  II. 

234.  Recomposition  of  White  Light.  — The  seven  colors, 
violet,  indigo,  blue,  green,  orange,  yellow,  and  red,  are  com- 
monly spoken  of  as  the  primary  colors.  It  is  more  correct, 
however,  to  accept  the  number  of  colors  as  infinite,  for 


LIGHT. 


341 


Fig.  283. 


they  pass  from    one   end  of  the   spectrum  to  the   other 
through  imperceptible  shades. 

White,  then,  is,  strictly  speaking,  not  a  color,  but  a  com- 
bination of  all  colors:  this  is  easily  shown  by  another  ex- 
periment. After  decomposing  light  by  passing  it  through 
a  prism,  we  can  combine  the  separated  colors  and  form 
white  light  again.  The  manner  in  which  this  is  done  is 
represented  in  Fig.  288.  The  beam  of  light,  after  passing 
through  the  prism  S  A  A', 
instead  of  proceeding  in 
the  direction  indicated 
by  the  dotted  lines  to 
form  the  spectrum,  is 
made  to  pass  through  the 
prism  S'B  B',  placed  in  a 
reversed  position,  and  its 
rays  are  refracted  so  as  to  assume  their  original  relation, 
making  a  white  beam,  M.  Here  the  second  prism  counter- 
acts the  effect  of  the  first,  because  its  position  is  exactly 
the  reverse.  Similar  results  are  obtained  by  substituting  a 
converging  lens  for  the  second  prism. 

Sir  Isaac  Newton,  who  first  experimented  on  this  subject,  considered 
the  decomposition  and  the  recomposition  of  light  as  affording  proof  that 
white  light  contains  seven  colors.  He  tried  various  experiments  to  prove. 

the  same  thing.  Thus,  he  mingled  to- 
gether intimately  seven  powders  hav- 
ing the  seven  prismatic  colors,  and  found 
that  the  mixture  had  a  grayish-white  as- 
pect. He  also  painted  a  circular  board 
with  these  colors,  and  found  that  on  whirl- 
ing it  so  rapidly  that  the  colors  could  not 
be  distinguished  the  whole  board  appeared 
to  be  white.  In  order  to  succeed  perfect- 
ly in  this  experiment  a  certain  proportion 
between  the  colors  must  be  observed,  MS 
indicated  in  Fig.  289.  A  very  prettv  wav 
P 


342  NATURAL   PHILOSOPHY. 

of  illustrating  the  composition  of  light  is  to  spin  a  top  painted  in  this  way. 
When  the  top  whirls  rapidly  it  is  white,  but  as  it  slackens  its  motion  the 
seven  colors  appear. 

The  names  of  these  seven  so-called  elementary  colors  can  be  easily  re- 
membered by  observing  that  the  initial  letters  spell  the  meaningless  but 
pronounceable  word  VIBGYOR ;  thus : 

V    I  B  G    Y    O    R 

i     n  1  re      re 

0  cl  u  e      1      ad 

1  i  e  e      In 
eg  nog 
to  we 

235.  Colors  of  Objects. — The  color  of  any  object  depends 
upon  the  manner  in  which  it  reflects  light.  Thus,  if  it  be 
red,  it  reflects  the  red  rays  of  the  spectrum,  absorbing  the 
other  rays ;  and  if  it  be  green,  it  reflects  the  green  rays, 
etc.  If  it  reflect  all  the  colors  together,  it  is  white ;  and  if 
it  reflect  none,  or  almost  none,  of  the  light,  it  is  black. 

You  can  readily  see  why  the  color  of  an  object  varies 
with  the  kind  of  light  that  falls  upon  it.  If  an  object 
which  is  red  in  sunlight  be  exposed  to  a  yellow  light,  such 
as  a  yellow  flame,  or  to  sunlight  that  has  passed  through 
a  yellow-colored  glass  or  curtain,  it  loses  its  red  color, 
for  there  are  no  red  rays  to  be  reflected  to  our  eyes.  A 
person  exposed  to  such  a  light  has  a  deathlike  paleness, 
the  lips  and  skin  losing  entirely  their  red  color.  This  effect 
can  be  witnessed  at  any  time  by  mixing  alcohol  with  a  lit- 
tle salt  on  a  plate  and  setting  fire  to  it,  or  by  throwing  salt 
into  a  coal  fire :  the  other  lights  in  the  room  should  be  re- 
moved to  obtain  the  full  effect.  This  explains  also  why 
the  colors  of  goods  examined  in  the  evening,  especially  by 
candle-light,  often  differ  somewhat  from  those  which  they 
have  in  the  day. 

In  some  substances  the  colors  are  changeable  with  varying  positions, 
though  the  light  be  the  same.  This  is  often  seen  in  shells  and  minerals, 
as  well  as  in  some  fabrics,  as  changeable  silk.  This  is  owing  to  the  ar- 


LIGHT. 


343 


rangement  of  the  particles,  by  which  variety  in  reflection  results  from 
changes  of  position. 

236.  The  Rainbow. — There  is  no  more  gorgeous  display 
of  colors  than  that  sometimes  seen  in  the  clouds  at  morn- 
ing or  evening.  These  colors  are  occasioned  simply  by 
refractions  and  reflections  in  the  minute  vesicles  (§  179) 
of  which  the  clouds  are  composed.  When,  however,  the 
moisture  is  condensed  into  drops,  a  still  more  magnificent 
spectacle  results — the  rainbow. 

The  rainbow  is  produced  by  the  action  of  light  on  the 
drops  of  falling  rain;  both  reflection  and  refraction  are  con- 
cerned in  its  formation.  Consider,  for  example,  that  which 
takes  place  in  a  single  drop,  represented  in  Fig.  290.  The 
sunbeam,  S,  entering  the  drop 
at  A,  is  refracted,  and  passes 
to  B,  at  the  farther  side  of  the 
drop.  Here  a  portion  of  it  is 
lost  by  its  proceeding  on  in 
the  line  B  C.  The  remainder 
is  reflected  to  D,  and  passes 
to  E,  being  again  refracted  as 
it  passes  out  into  a  rarer  medium,  the  air,  thus  produc- 
ing a  single  reflection  and  two  refractions.  But  in  the 
second  bow,  which  is  sometimes  formed,  there  are  two  re- 
flections as  well  as  two  refrac- 
tions, as  represented  in  Fig. 
291.  The  beam  of  light,  S, 
from  the  sun  enters  the  drop 
at  A,  is  refracted,  and  passes 
to  B.  Here  a  portion  pro- 
ceeds on  in  the  direction  B  C ; 
the  other  portion  is  reflected 

to  D.     Some  light  is  again  lost  by  proceeding  in  the  line 
D  E ;  that  which  remains  is  reflected  to  F.    This  shows  why 


344 


NATUKAL   PHILOSOPHY. 


the  second  bow  is  not  so  bright  as  the  primary  one :  in  the 
latter  there  is  but  one  reflection  in  each  drop,  and  there- 
fore loss  of  light  occurs  at  but  one  point;  while  in  the  for- 
mer there  are  two  reflections,  and  therefore  loss  of  light  at 
two  points. 

237.  Circumstances  under  -which  Rainbows  are  Seen. — A 
rainbow  is  seen  when  the  spectator  stands  between  the  sun 
and  falling  rain.  This  can  rarely  be  done  except  in  the 
latter  part  of  the  day.  It  sometimes,  though  very  rarely, 
happens  that  a  shower  passes  from  the  east  to  the  west  .in 
the  morning,  and  then  a  rainbow  can  be  seen  in  the  west. 
:.  292  is  intended  to  show  under  what  circumstances  a 


Fig.  292. 


LIGHT.  345 

rainbow  is  seen.  Let  a  horizontal  line  be  drawn  from  O, 
the  observer,  to  P,  a  point  directly  under  the  middle  point 
of  the  arch.  If  this  line  were  prolonged  behind  the  observ- 
er, it  would  extend  precisely  in  the  direction  of  the  sun. 
That  is,  the  sun  is  directly  opposite  the  middle  of  the  bow. 
Now,  if  the  drop  at  A  reflect  a  red  ray  to  the^  eye  of  the 
spectator,  all  other  drops  similarly  situated  in  the  arch  will 
reflect  red  rays.  If  B  reflect  a  green  ray,  all  other  drops 
similarly  situated  will  do  the  same.  And  so  of  C,  reflect- 
ing the  violet  ray.  For  the  sake  of  clearness  only  three 
reflections  are  represented,  but  the  same  is  true  of  all  the 
seven  colors.  In  the  secondary  bow  the  arrangement  of  the 
colors  is  reversed,  the  red  being  at  the  inner  part  of  the  bow 
and  the  violet  at  the  outer  part.  The  double  reflections  are 
manifest  in  the  drops  D,  E,  and  F.  That  which  we  have 
described  as  taking  place  in  a  few  drops  takes  place  in 
countless  multitudes  of  them  in  forming  the  bow.  Since 
the  exact  position  of  the  rainbow  depends  not  only  upon 
the  direction  of  the  rays  of  the  sun,  but  also  upon  the  posi- 
tion  of  the  spectator,  it  is  clear  that  no  two  spectators  see 
precisely  the  same  bow,  for  the  drops  that  form  it  for  the 
one  are  not  the  same  as  those  forming  it  for  the  other.  This 
is  very  obvious  if  the  two  be  quite  distant  from  each  other- 
but  it  is  equally  true  if  they  are  very  near  together,  although 
in  this  case  the  bow  seen  by  one  spectator  would  be  very 
nearly  coincident  with  that  seen  by  the. other.  It  is  also 
true  that  the  rainbow  of  one  moment  is  not  the  rainbow  of 
the  next;  for  since  the  drops  that  reflect  it  are  falling  drops, 
there  must  be  a  constant  succession  of  them  in  any  part  of 
the  bow. 

Colors  in  Dew-drops  and  Ice  Crystals. — We  often  see  something  very 
analogous  to  the  rainbow  in  the  dew.  If  we  look  at  the  dew-drops  when 
standing  with  our  backs  to  the  rising  sun,  we  see  all  the  colors  of  the  rain- 
bow glistening  everywhere  before  us,  as  if  the  grass  were  filled  with  gems  of 


346  NATURAL   PHILOSOPHY. 

every  hue.  This  is  occasioned  by  refraction  and  reflection  in  drops  of  water, 
and  the  resemblance  fails  only  in  the  regularity  of  arrangement  which  the 
rainbow  presents.  We  see  the  same  thing  also  if  the  ground  is  strewed  with 
bits  of  ice  which  have  fallen  from  the  branches  of  the  trees,  and  the  sun 
shines  aslant  upon  them. 

At  the  foot  of  Niagara  Falls,  when  the  sun  shines  favorably  upon  the  dense 
mists,  perfectly  circular  rainbows  are  seen,  the  spectator  viewing  the  mist 
below  him  as  well  as  that  above  him.  The  colorless  halos  seen  around  the 
moon  are  the  result  of  reflections  of  light  from  the  external  surfaces  of  drops 
of  water,  without  refractions. 

238.  Heat,  Light,  and  Chemical  Rays. — According  to  the 
undulatory  theory  (§  211),  light  consists  of  vibrations  of 
an  all-pervading  imponderable  ether;  and  these  vibrations 
have  a  wave-like  character.  It  has  been  found  that  the 
rays  of  different  colored  light  vary,  both  as  regards  the 
length  of  the  waves  and  their  velocity.  Thus,  the  red 
waves  are  long  and  the  violet  waves  short.  Expressed  in 
figures,  the  lengths  are  so  small  as  to  be  entirely  beyond 
our  conception ;  and  yet  by  delicate  operations,  which  we 
cannot  explain  in  this  elementary  work,  these  marvellously 
small  distances  have  been  determined  experimentally.  The 
number  of  vibrations  per  second  is  so  enormous,  on  the 
other  hand,  as  to  be  equally  beyond  our  comprehension. 
The  length  of  the  waves  constituting  red  light  is  620  mill- 
ionths  of  a  millimetre,  and  the  ether  producing  this  light 
makes  514,000,000,000,000  vibrations  in  one  second.  Pass- 
ing from  red  to  violet  through  the  spectrum,  the  waves  of 
each  successive  color  are  shorter  and  shorter,  and  the  num- 
ber of  vibrations  of  the  ether  greater  and  greater,  until  we 
reach  the  violet,  for  which  the  following  figures  have  been 
determined:  violet  light  has  waves  423  millionths  of  a  mil- 
limetre in  length,  and  makes  752,000,000,000,000  vibrations 
per  second. 

In  this  respect  there  is  an  analogy  with  both  sound  and 
heat.  As  mentioned  in  §  148,  the  vibrations  which  produce 


LIGHT. 


347 


sound  are  from  16  to  38,000  per  second,  the  pitch  depend- 
ing on  the  velocity.  The  temperature  also  depends  upon 
the  rapidity  of  the  vibrations  of  the  molecules  of  the  bod- 
ies and  of  the  surrounding  ether;  and  now  we  learn  that 
the  colors  depend  upon  similar  causes.  The  vibrations  of 
the  ether  which  produce  light  are,  however,  not  the  most 
rapid  known :  other  rays  exist  which  yield  neither  heat 
nor  light,  and  the  motions  of  which  are  far  more  rapid. 
The  existence  of  these  invisible  rays  is  proved  by  the 
chemical  effects  they  are  capable  of  producing ;  they  are 
consequently  usually  called  chemical  rays.  The  term  acti- 
nism is  also  used  to  express  this  form  of  force. 

When  the  combined  rays  of  heat,  light,  and  actinism  are 
passed  through  a  prism,  they  are  dispersed  in  the  manner 
shown  in  Fig.  293,  the  short  rapid  waves  bending  more 


Fig. 293. 


than  the  long  slow  ones.  Between  II  and  R  on  the  screen 
are  invisible  heat  rays;  between  R  and  V  the  colors  in 
their  order;  and  between  V  and  C  the  chemical  rays, 
which  are  powerless  to  affect  our  eyes,  but  affect  strongly 
certain  chemical  substances.  It  is  these  rays  which  do  the 
work  in  photography.  The  chemical  effects  of  light  will 
be  described  in  Part  II.  of  this  series  —  Chemistry.  The 
heat,  light,  and  chemical  rays  overlap  each  other,  and  are 
not  terminated  by  sharp  lines,  as  you  might  judge  from 


348 


NATURAL   PHILOSOPHY. 


Fig.  293.  The  manner  in  which  they  are  distributed  is 
shown  in  Fig.  294,  in  which  the  height  of  the  curved  lines 
represents  the  intensity  of  the  rays  at  each  point;  the 
greatest  heating  effect  being  outside  the  red  end  of  the 
visible  spectrum,  and  the  greatest  chemical  effect  residing 
beyond  the  violet  at  the  opposite  end. 


#     O     •<     Q     »     —     < 

*  1  I  1  s  |  f 

I     I     ?     •      p    r 
Fig.  ZW. 

240.  Crookes's  Radiometer.  —  The  intimate  connection  between 
heat  and  light  is  manifested  in  the  operation  of  an  exceedingly  curious  lit- 
tle instrument  recently  invented  by  William  Crookes,  of  London,  which 
he  calls  a  radiometer.  It  consists  of  two  cross-arms  of  aluminum,  \vhich 
carry  at  their  ends  little  thin  plates  of  mica  blackened  on  one  side.  The 
cross-arms  are  attached  by  melting  to  a  vertical  glass  axis,  which  rests 
below  on  a  steel  point,  on  which  it  turns  freely. 
The  whole  is  placed  in  a  glass  globe,  in  which 
a  vacuum  has  been  made,  so  as  to  withdraw  the 
resistance  of  the  air.  In  this  condition,  when 
exposed  to  the  light  of  a  candle,  daylight,  etc., 
falling  on  one  side  of  the  apparatus,  the  arms 
commence  to  revolve,  the  clean  side  of  the  mica 
moving  towards  the  light,  while  the  velocity  of 
rotation  is  more  rapid  in  proportion  as  the  light 
is  stronger.  Thus,  for  instance,  when  placed 
in  the  sunshine,  the  rapid  rotation  will  at  once 
be  retarded  when  a  cloud  passes  over  the  sun ; 
hence  it  is  that  Crookes  called  the  instrument 
a  "radiometer,"  attributing  its  motion  to  the 
reaction  of  the  reflection  of  the  rays  of  light. 
This  explanation  was  not  adopted  by  the  ma- 
jority of  savans,  who  did  not  see  any  foundation  for  it,  but  found,  by  care- 
ful experiment  and  consideration,  that  the  motion  was  due  to  the  action  of 
heated  rays  in  a  rarefied  gas.  If,  indeed,  it  were  due  to  the  reaction  of 


Fig.  295. 


LIGHT.  349 

reflection,  the  polished  side  should  move  from  the  light,  and  the  blackened 
side  (which  absorbs  light)  should  move  towards  it ;  but  the  contrary  is  the 
case,  and  the  motion  is  evidently  due  to  the  heat  developed  in  the  black- 
ened side  of  the  mica  in  the  thin  layer  of  rarefied  air  there,  which  by  its 
expansion  presses  against  this  side,  and,  thus  acting  as  a  dilating  spring, 
pushes  it  forward. 

Others  add  to  this  that  a  perfectly  dry  vacuum  is  unattainable,  and  that 
therefore  the  watery  vapor  always  condensed  on  the  black  pulverulent  sur- 
face of  the  mica  is  evaporated  by  the  least  heat ;  and  when  emitted,  by  its 
reaction  pushes  the  black  surface  forward,  the  same  as  the  air  in  the  for- 
mer explanation. 


QUESTIONS. 

211.  What  was  Newton's  theory  of  the  nature  of  light  ?  What  is  the  un- 
dulatory  theory?  Show  the  difference  between  sound  vibrations  and  those 
of  light.  Explain  the  terms  wave-length  and  amplitude.  Show  the  anal- 
ogy between  heat  and  light. — 212.  When  is  a  body  luminous?  What  are 
the  sources  of  light  ?  What  is  said  of  chemical  action  as  a  source  of  light  ? 
— 213.  Define  opaque  and  transparent.  How  may  you  see  that  light  moves 
in  straight  lines?  State  various  familiar  recognitions  of  this  fact. — 214. 
Illustrate  the  fact  that  the  intensity  of  light  is  inversely  as  the  square  of  the 
distance.  How  are  lights  of  different  intensities  compared  ?  What  is  a 
photometer? — 215.  What  is  said  of  the  velocity  of  light  in  regard  to  ordi- 
nary distances  ?  How  long  is  light  coming  from  the  sun  to  the  earth  ? 
What  is  said  of  the  light  coming  to  us  from  certain  stars?  Give  the  ob- 
servation of  Roemer  represented  in  Fig.  259. — 216.  What  is  said  of  the 
reflection  of  light?  What  of  its  reflection  in  relation  to  seeing?— 217. 
What  of  the  images  formed  in  mirrors  ?  Show  by  Fig.  2G1  why  the  image 
in  a  mirror  seems  to  be  at  the  same  distance  behind  it  that  the  object  is 
before  it. —  218.  Explain  the  operation  of  the  kaleidoscope. — 219.  What 
are  the  chief  kinds  of  curved  mirrors  ?  Explain  the  operation  of  a  concave 
mirror.  Explain  that  of  a  convex  mirror. — 220.  What  is  meant  by  the 
refraction  of  light  ?  Illustrate  its  refraction  in  passing  from  a  denser  into 
a  rarer  medium.  Then  from  a  rarer  into  a  denser.  How  is  the  refraction 
in  regard  to  a  perpendicular  in  the  two  cases?  What  is  said  of  the  refrac- 
tive power  of  different  substances  ? — 221.  Explain  dawn  and  twilight.  Ex- 
plain what  is  represented  in  Fig.  267. — 222.  What  are  mirages  ?  Describe 
the  mirage  which  occurred  at  Ramsgate.  Describe  that  seen  by  Captain 
Scoresby.  Relate  the  incident  which  occurred  at  New  Haven.  What  is 

P2 


350  NATURAL  PHILOSOPHY. 

said  of  mirages  in  deserts  ? — 223.  Explain  what  is  meant  by  the  visual 
angle.  Explain  Fig.  269.— 224.  What  are  lenses?  What  are  the  differ- 
ent kinds  ?  What  is  the  difference  of  effect  in  convex  and  concave  lenses  ? 
Explain  the  effect  of  a  convex  lens  on  the  visual  angle.  Upon  what  does 
the  magnifying  power  of  a  lens  depend? — 225.  What  is  said  of  micro- 
scopes and  telescopes? — 226.  Describe  and  explain  the  magic  lantern. — 
227.  Describe  and  explain  the  camera  obscura.  Describe  the  arrangement 
of  a  camera  for  sketching. — 228.  How  is  the  eye  like  a  camera  ?  Describe 
the  arrangement  of  the  parts  of  the  eye.  Show  more  fully  the  analogy 
between  the  eye  and  a  camera.  What  is  said  of  the  influence  of  the 
cornea  on  the  light?  —  229.  Show  what  is  required  for  distinct  vision. 
Show  why  it  is  that  -objects  brought  very  near  the  eye  are  not  seen  dis- 
tinctly. What  is  said  of  the  microscope  ?  Explain  the  difficulty  in  the 
near-sighted.  In  the  far-sighted. — 230.  How  can  you  show  that  the  im- 
ages of  objects  in  the  retina  are  inverted  ?  Give  in  full  what  is  said  of 
explanations  of  the  fact  that  we  see  objects  erect,  notwithstanding  this 
inversion. — 231.  Explain  single  vision.  By  what  simple  experiment  can 
you  show  the  explanation  of  single  vision  to  be  correct?  What  is  said 
of  squinting? — 232.  Explain  the  stereoscope.  What  is  said  of  distinct 
impressions  on  the  retina?  —  233.  Explain  the  thaumatrope.  Name 
other  contrivances  for  illustrating  the  persistence  of  vision.  —  234.  Ex- 
plain the  path  of  a  ray  of  light  through  a  piece  of  glass  having  parallel 
surfaces.  What  is  a  prism  ?  What  course  does  light  take  in  passing 
through  a  prism  ?  Why  ?  What  is  a  spectrum  ?  Explain  the  experiment 
showing  the  dispersion  of  white  light. — 235.  What  is  said  of  the  recompo- 
sition  of  decomposed  light  ?  Give  the  illustration  of  the  powders.  The 
circular  board.  The  top. — 236.  What  is  said  of  the  colors  of  substances  ? 
What  of  the  variations  of  these  colors  in  different  lights  ?  What  of  vari- 
ations with  varying  positions  ? — 237.  What  of  the  colors  of  clouds  ?  Ex- 
plain the  formation  of  the  first  rainbow  by  Fig.  290.  Explain  the  forma- 
tion of  the  second  bow  by  Fig.  291. — 238.  What  is  said  of  the  circum- 
stances under  which  rainbows  are  seen  ?  Explain  in  full  the  formation  of 
the  two  bows  as  illustrated  by  Fig.  292.  What  is  said  of  the  bow  as  seen 
by  different  persons,  and  at  different  moments  by  the  same  person  ?  What 
of  rainbow  hues  in  dew-drops  and  ice  crystals  ? — 239.  In  what  respect  do 
rays  of  different  colors  differ  ?  What  is  said  of  the  analogy  of  sound,  heat, 
and  light  ?  What  are  chemical  rays  ?  At  which  end  of  the  solar  spectrum 
are  they  situated  ?  Show  how  heat,  light,  and  chemical  rays  are  distribu- 
ted in  the  solar  spectrum.  —  240.  Describe  Crookes's  radiometer.  What 
theories  have  been  advanced  to  account  for  its  motion  ? 


ELECTRICITY.  351 


CHAPTER  XVIII. 

ELECTRICITY. 

241.  The  Effects  of  Electricity. — The  ancient  Greeks,  so 
long  as  2500  years  ago,  observed  that  when  amber  was 
rubbed  with  woollen  cloth  it  acquired  the  singular  proper- 
ty of  attracting  light  substances — such  as  bits  of  paper, 
shreds  of  cloth,  etc.  The  Greek  word  for  amber  being  elec- 
tron, the  power  thus  excited  has  been  called  electricity. 
Little  else  was  known  of  electricity  until  about  280  years 
ago,  when  Dr.  Gilbert,  physician  to  Queen  Elizabeth,  showed 
that  other  substances  besides  amber — such  as  glass,  wax, 
sulphur,  etc — possessed  similar  properties.  Since  then  many 
philosophers  have  studied  the  remarkable  phenomena  ex- 
hibited by  electricity;  and,  within  comparatively  recent 
times,  it  has  become  an  important  branch  of  natural  phi- 
losophy. 

The  precise  nature  of  electricity  has  not  been  definitely 
determined.  Various  theories  have  been  offered  to  explain 
its  existence  and  effects,  of  which  we  shall  have  more  to  say 
later  on.  Meanwhile  let  us  examine,  experimentally,  some 
of  the  facts  for  the  explanation  of  which  the  theories  have 
been  propounded. 

The  apparatus  required  to  illustrate  the  fundamental 
facts  of  electricity  is  very  simple  and  inexpensive.  Any 
one  provided  with  (l)  a  silk  handkerchief  or  a  woollen 
cloth,  (2)  a  piece  of  sealing-wax,  (3)  a  glass  tube,  (4)  some 
round  pieces  of  cork  or  of  elder  pith  hanging  from  any  sup- 
port by  means  of  silk  threads^  (5)  a  common  fire-poker,  and 


352  NATURAL  PHILOSOPHY. 

(6)  a  few  wine-glasses,  can  easily  make  himself  acquainted 
with  a  large  number  of  the  effects  of  electricity. 

If  you  rub  the  stick  of  sealing-wax  with  the  silk  hand- 
kerchief, and  then  hold  it  quite  close  to  some  bits  of  paper 
or  shreds  of  cotton  cloth,  the  latter  will  be  attracted  to  the 
sealing-wax,  springing  a  short  distance  towards  it,  and  then 
adhering  feebly  to  its  surface.  Amber  and  many  other 
substances  act  similarly.  As  already  mentioned,  this  ex- 
periment is  more  than  2500  years  old. 

Again,  if  you  rub  the  well-dried  glass  tube  or  rod  with 
the  silk  handkerchief,  as  shown  in  Fig.  296,  and  then  hold 


Fig.  290. 

it  near  light  articles,  such  as  feathers,  lint,  bits  of  paper, 
etc.,  the  glass,  also,  will  attract  them.  Under  favorable 
circumstances  pith  balls  of  some  size  will  spring  to  the 
glass  tube,  as  represented  in  Fig.  297. 

The  force  thus  developed  by  friction  is  called  electricity, 
and  the  tube,  ball,  or  other  object  in  which  the  electrical 
action  is  excited  is  said  to  be  .electrified. 


ELECTRICITY. 


353 


Fig.  297. 

We  learn  from  the  experiments  described  that  one  of  the 
most  common  effects  of  electricity  is  attraction;  another 
simple  experiment  will  show  that  sometimes,  however,  repul- 
sion is  produced  by 
electricity.  Suspend 
the  pith  ball  by  a 
silk  thread,  and  at- 
tach it  to  a  glass  rod 
inserted  into  a  bot- 
tle (filled  with  sand 
to  make  it  stand 
firm);  and  then, hav- 
ing rubbed  the  glass 
tube  with  the  silk 
handkerchief,  bring 
it  near  the  ball,  as 
represented  in  Fig. 
298.  The  pith  ball 
will  be  drawn  tow- 
ards the  tube  and  fol- 
low it  when  the  lat- 
ter is  moved  about ; 
if,  however,  the  ball 

be  allowed  to  touch  the  tube,  then  in  an  instant  the  ball 
will  be  repelled  by  it,  and  when  the  tube  is  moved  the 


354  NATURAL   PHILOSOPHY. 

ball  will  try  to  escape  touching  it,  and  run  away  from  it 
as  if  endowed  with  life. 

After  the  ball  has  touched  the  electrified  tube,  it  be- 
comes itself  electrified,  and  will  attract  any  light  object 
(as  another  ball)  brought  near  it.  So  soon,  however,  as  the 
two  balls  have  touched  each  other,  they  will  repel  mutual- 
ly, just  as  the  glass  tube  and  first  ball  do  after  contact. 
If  either  electrified  ball  be  touched  with  the  finger,  it  loses 
its  electricity. 

The  following  experiments  will  illustrate  another  point. 
Support  the  iron  poker  on  two  wine-glasses,  and  attach  two 
pith  balls  by  linen  threads,  or  a  fine  wire  to  one  end  of  the 
poker ;  then  touch  the  other  end  with  the  excited'  glass 
tube,  and  the  two  balls  will  fly  apart,  showing  that  the 
metallic  rod  has  permitted  the  passage  of  the  electricity 
throughout  its  length.  By  this  we  learn  that  some  sub- 
stances act  as  conductors  of  electricity,  while  others  are 
non-conductors.  The  wine-glasses,  being  non-conductors^ 
form  a  suitable  support  for  the  poker  and  prevent  the  es- 
cape of  the  electricity. 

All  these  experiments  can  be  made  with  the  silk  hand- 
kerchief itself  as  well  as  with  the  glass  tube.  The  effects, 
however,  are  far  more  feeble,  and  much  care  is  necessary  to 
obtain  them.  This  singular  difference,  moreover,  is  notice- 
able: objects  which  are  attracted  by  the  glass  are  repelled 
by  the  silk,  and  those  repelled  by  the  glass  are  attracted 
by  the  silk.  One  of  the  rubbing  bodies  appears  to  gain 
that  which  the  other  loses. 

Suppose,  again,  you  rub  the  stick  of  sealing-wax  with  the 
silk  or  woollen  cloth  and  hold  it  near  a  pith  ball  previously 
electrified  by  the  glass  tube;  the  ball  will  be  attracted; 
and  if  you  bring  a  glass  tube,  rubbed  as  before,  near  a  ball 
electrified  by  the  sealing-wax,  this  ball  will  also  be  attract- 
ed. Lastly,  if  you  electrify  one  pith  ball  by  means  of  seal- 


ELECTRICITY.  355 

ing-wax  and  the  other  by  means  of  glass,  the  two  balls  will 
attract  each  other.  There  seem,  then,  to  be  two  kinds  or 
states  of  electricity  having  attraction  for  each  other,  but 
each  repelling  itself. 

242.  Theories  of  Electricity. —  To  account  for  these  and 
other  phenomena,  several  theories  have  been  proposed.  Ac- 
cording to  one,  electricity  is  an  imponderable  fluid  exist- 
ing in  all  bodies  to  a  greater  or  less  degree,  according  to 
their  capacity  for  electricity.  While  a  body  is  in  its  usual 
state  there  is  no  manifestation  of  electricity.  The  fluid  is 
in  a  quiescent  condition,  because  its  particles  are  prevented 
from  repelling  each  other  by  the  attraction  which  exists 
between  them  and  the  particles  of  the  substance.  But  this 
quiescence  can  be  disturbed  by  friction  and  other  causes. 
Thus,  if  a  glass  rod  be  rubbed  with  a  piece  of  silk,  the  nat- 
ural equilibrium  is  disturbed,  the  glass  having  an  excess 
and  the  cloth  a  deficiency  of  electricity.  The  glass  is  there- 
fore said  to  be  positively  and  the  cloth  negatively  electri- 
fied. The  equilibrium  can  be  restored  in  the  case  of  a  posi- 
tively electrified  body  by  having  its  excess  drawn  off,  and 
in  the  case  of  a  negatively  electrified  body  by  having  its 
deficiency  made  up  by  receiving  electricity  from  other  bod- 
ies. This  was  Franklin's  view,- and  is  known  as  the  single- 
fluid  theory  ;  opposed  to  this  is  the  two-fluid  theory,  which 
supposes  the  existence  of  two  electric  fluids  which,  like  an 
acid  and  an  alkali,  neutralize  each  other  when  present  in 
equal  quantity,  but  which  have  a  strong  attraction  or  affin- 
ity when  separated  ;  while,  on  the  other  hand,  the  particles 
of  either  fluid  are  repellent  to  each  other.  When  friction 
or  other  cause  excites  electrical  action,  the  two  fluids  are 
separated  and  become  evident  to  the  senses.  This  theory 
seems  to  explain  the  fact  that  sealing-wax  and  glass,  or  the 
glass  and  a  silk  handkerchief,  develop  respectively  electric- 
ities of  a  different  character;  and  hence  the  electricity  ex- 


356  NATURAL   PHILOSOPHY. 

cited  by  friction  of  sealing-wax  received  the  name  resinous 
electricity,  and  that  of  glass  vitreous  electricity.  The 
names  positive  and  negative  are  also  applied  to  vitreous 
and  resinous  electricity,  these  appellations  being  borrowed 
from  the  single-fluid  theory  of  Franklin. 

As  we  have  already  seen  in  the  case  of  both  heat  (§  165) 
and  light  (§  211),  the  theories  based  upon  the  existence  of 
imponderable  fluids  have  little  by  little  been  discarded,  and 
eventually  gave  way  to  the  dynamical  theory,  or  theory  of 
motion. 

In  the  same  way  the  fluid  theories  of  electricity  have 
been  abandoned,  and  it  is  now  regarded  as  a  mode  of  force 
operating  on  ordinary  matter,  the  molecules  of  which  it 
arranges  in  a  definite  direction.  Moreover,  it  is  converti- 
ble into  the  other  modes  of  force — heat,  light,  magnetism, 
and  chemical  attraction. 

Many  of  the  terms  commonly  used  in  explaining  the  facts 
of  electricity  are,  however,  derived  from  the  fluid  theory, 
and  still  maintain  their  hold  on  the  language  of  electri- 
cians. Thus  the  expressions  "  currents,"  "  charged  with 
the  electric  fluid,"  are  remains  of  the  abandoned  theories. 

243.  Positive  and  Negative  Electricity. — The  terms  posi- 
tive and  negative  are  retained  as  convenient  for  expressing 
the  character  of  the  electricity  developed.  Whether  one  or 
the  other  is  excited  depends  upon  the  nature  of  the  rubber. 
Thus,  smooth  glass  rubbed  with  woollen  c^oth  or  silk  will 
be  positively  electrified ;  while  if  it  be  rubbed  upon  the 
back  of  a  cat,  it  will  exhibit  negative,  or  resinous,  electric- 
ity. If  a  resin,  as  gumlac  or  sealing-wax,  be  rubbed  with 
silk  or  woollen  cloth,  it  will  be  charged  with  negative,  or 
resinous,  electricity ;  but  if  it  be  rubbed  with  sulphur,  it 
will  be  charged  with  vitreous,  or  positive,  electricity.  The 
terms  vitreous  and  resinous  are  therefore  incorrect,  for  they 
are  based  upon  the  idea  that  one  kind  of  electricity  is  always 


ELECTRICITY.  357 

excited  on  glass,  with  whatever  substance  the  friction  may 
be  made,  and  that  the  other  kind  is  always  excited  on  resins. 
The  most  decided  illustration  of  the  incorrectness  of  these 
terms  we  have  in  the  fact  that  while  smooth  glass  rubbed 
with  silk  or  woollen  cloth  becomes  charged  with  positive 
(vitreous)  electricity,  roughened  glass  rubbed  witli  the  same 
gives  us  negative  (resinous)  electricity.  Below  is  a  list 
of  substances  any  one  of  which  develops  positive  electrici- 
ty when  rubbed  with  any  substance  below  it,  and  negative 
when  rubbed  with  any  substance  above  it : 

1.  Cat-skin.  7.  Silk. 

2.  Polished  glass.  8.  Sealing-wax. 

3.  Woollen  cloth.  0.  Amber. 

4.  Feathers.  10.  Roughened  glass.     . 

5.  Wood.  11.  Sulphur. 

6.  Taper. 

As  a  result  of  experiments  mentioned  in  the  latter  part 
of  §  241,  and  of  the  facts  just  mentioned,  the  following  law 
has  been  established:  substances  charged  with  like  elec- 
tricities repel  each  other,  while  those  charged  with  unlike 
electricities  attract  each  other.  By  like  electricities  is  meant 
those  having  similar  names,  whether  the  terms  used  are 
positive,  negative,  vitreous,  or  resinous.  Thus,  a  pith  ball 
negatively  electrified  repels  another  ball  charged  with  the 
same  kind  of  electricity,  and  attracts  one  positively  elec- 
trified. 

244.  Electricity  Everywhere  Active. — Electricity  exists  in  all 
substances,  each  having  its  own  capacity  for  it,  but  in  the  usual  condition  of 
substances  the  electricity  is  in  a  state  of  equilibrium,  and  therefore  of  quiet. 
We  see  this  quiet  disturbed  during  a  thunder-storm,  when  we  rub  glass  or 
silk,  or  a  cat's  back,  or  when  we  work  an  electrical  machine.  But  the  ac- 
tive state  of  electricity  is  not  limited  to  such  palpable  demonstrations  as  these. 
Electricity  is  undoubtedly  in  action  everywhere  and  always,  although  we 
can  seldom  appreciate  and  measure  its  action.  Wherever  there  is  motion 
the  equilibrium  of  electricity  is  disturbed,  and  there  is  a  consequent  return 


358  NATURAL   PHILOSOPHY. 

to  this  equilibrium.  And  this  change  from  the  one  state  to  the  other  must 
be  the  constant  cause  of  important  changes  and  operations  in  the  world 
around  us,  and  in  our  own  bodies.  Let  us  look  at  some  of  the  indications 
of  this  universality  of  electrical  action.  Friction  constantly  awakens  it. 
The  friction  of  the  belts  upon  the  drums  in  cotton  factories  develops  it  quite 
freely.  Every  stroke  of  India-rubber  upon  paper  as  you  erase  a  pencil- mark 
excites  electricity.  The  blowing  of  air  upon  glass  does  the  same,  as  well 
as  the  blowing-off  of  steam  from  an  engine.  Electricity  has  been  excited 
even  upon  ice  by  rubbing  it  when  cooled  down  to  —25°  Centigrade.  Ex- 
periments upon  the  air  have  shown  that  there  is  usually  some  free  electricity 
in  it,  the  atmosphere  being  generally  in  a  positive  state,  especially  when  the 
air  is  dry  and  clear.  It  is  constantly  generated  from  one  source  and  an- 
other. It  is  generated  everywhere  by  evaporation.  Every  gust  of  wind, 
causing  friction  of  the  particles  of  the  air  upon  various  substances,  gener- 
ates it.  Chemical  action,  as  you  will  learn  in  another  chapter,  generates  it 
everywhere.  It  is  produced  also  in  the  operations  of  life,  and  in  some  an- 
imals there  are  special  organs— electrical  batteries— for  the  generation  of 
this  mysterious  force. 

245.  Conductors  and  Non-Conductors. — Electricity  passes 
over  the  surface  of  some  substances  very  readily;  while 
over  others  it  moves  with  very  great  difficulty,  and  there- 
fore very  slowly  and  sparingly.  The  former  are  termed 
conductors,  and  the  latter  non-conductors.  As  in  the  case 
of  heat,  there  are  no  perfect  non-conductors.  The  best  of 
all  the  conductors  are  the  metals,  those  least  liable  to  oxida- 
tion being  the  most  perfect.  Next  come  charcoal,  water, 
living  substances,  flame,  moist  earth,  ice.  The  best  non- 
conductors are  dry  air  and  gases.  Then  come  gumlac  and 
gutta-percha,  amber,  resins,  sulphur,  glass,  silk,  wool,  hair, 
feathers,  cotton,  paper,  leather,  porcelain,  marble,  and  oils, 
very  nearly  in  the  order  named.  Non-conductors  are  some- 
times called  insulators  (from  the  Latin  word  insula,  an  isl- 
and), as  they  serve  to  confine  electricity  within  certain 
bounds  and  prevent  its  escaping.  Thus  in  the  experiments 
with  pith  balls  already  cited,  the  silk  threads  by  which  they 
are  suspended  prevent  the  escape  of  electricity.  The  glass 


ELECTRICITY. 


359 


knobs  on  which  the  wires  of  the  telegraph  rest  are  insula- 
tors, preventing  the  electric  fluid  from  escaping  down  the 
poles  into  the  ground. 

Electrics  and  Non- Electrics. — It  will  be  observed,  on  looking  over  the 
list  of  conductors  and  non-conductors,  that  among  the  non-conductors  are 
those  substances  in  which  electricity  is  easily  excited  by  friction,  such  as 
glass,  amber,  silk,  etc.  These  were  therefore  called  electrics.  The  con- 
ductors, on  the  other  hand,  were  called  non-electrics,  it  being  supposed  that 
electricity  could  not  be  excited  with  them.  But  this  has  proved  to  be 
incorrect.  Tor  example,  if  a  metal  be  insulated  by  being  placed  on  a  pil- 
lar of  glass  *or  of  gumlac,  so  that  the  electrjcity,  when  excited,  cannot  pass 
off  readily,  its  generation  can  be  made  manifest.  It  is  probably  true  that 
every  substance  is  more  or  less  an  electric,  it  being  difficult  to  make  this 
manifest  in  the  case  of  conductors,  because  the  electricity  passes  off  as  fast 
as  generated. 

246.  Electricity  Always  on  the  Surface. — There  is  a  mark- 
ed difference  between  heat  and  electricity  in  the  manner  in 
which  they  are  distributed.  Heat  pervades  all  the  particles 
of  substances,  and  by  conduction  spreads  through  them, 
while  electricity  in  its  ordinary  movements  operates  alto- 
gether on  the  surface.  A  hollow  ball,  therefore,  can  contain 
as  much  electricity  as  a  solid,  and  a  hollow  conductor  of 
electricity  is  just  as  ef- 
fectual as  a  solid  one. 
The  following  experi- 
ment exhibits  in  a  very 
striking  manner  this 
disposition  of  electric- 
ity to  occupy  the  sur- 
face alone.  Fig.  299 
represents  a  hollow 
metallic  ball  support- 
ed by  a  glass  stand, 
and  two  metallic  caps 
which  will  just  cover  Fig.  299. 


360 


NATURAL   PHILOSOPHY. 


the  ball,  having  non-conducting  handles,  of  either  glass  or 
gumlac.  Now,  after  having  charged  the  ball  with  elec- 
tricity, let  the  caps  held  by  the  insulating  handles  be  care- 
fully placed  over  the  ball.  On  withdrawing  them,  it  will 
be  found  that  the  electricity  of  the  ball  has  all  passed  to 
the  outer  surface  of  these  caps. 

The  illustrious  Faraday  devised 
another  simple  and  ingenious  ex- 
periment to  show  that  electricity 
does  not  penetrate  into  the  mass  of 
a  body.  Fig.  300  represents  a  con- 
ical sack  of  woollen  gauze  attach- 
ed to  a  metallic  ring,  which  is 
supported  on  an  insulating  stand. 
A  silk  thread  is  fastened  to  the 
point  of  the  cone  by  means  of 
which  the  bag  may  be  turned  in- 
side out.  When  the  conical  sack 
is  charged  with  electricity,  the 
outer  surface  only  is  found  to  be 
electrified ;  and  if  the  bag  be  turned  inside  out,  the  exterior 
will  again  be  the  only  portion  containing  electricity. 

Electricity  spreads  or  dis- 
tributes itself  uniformly  over 
the  surface  of  a  body  only 
when  that  body  has  the  form 
of  a  ball  or  sphere.  When  a 
body  having  the  oblong  form 
(shown  in  Fig.  301)  is  charged 
with  electricity,  the  greatest 
density  of  the  fluid  (borrow- 
ing a  phrase  from  the  fluid 
theory)  is  at  either  end.  This 
is  said  to  result  from  the  re-  Fig.  301. 


Fig. 300. 


ELECTRICITY.  361 

pulsion  of  the  fluid  in  the  central  part  of  the  body.  The 
signs  -f  and  —  placed  at  either  end  signify  that  one  end  is 
charged  with  positive  and  the  other  with  negative  electricity. 
This  method  of  arrangement  is  sometimes  called  polarity. 
When  a  body  smaller  at  one  end  than  at  the  other  is  charged 
with  electricity,  the  latter  accumulates  at  the  smaller  end ; 
and  if  this  be  a  point,  the  density  becomes  so  great  that 
the  electricity  is  forced  into  the  air  and  gradually  escapes. 
Hence,  apparatus  intended  to  confine  electricity  to  its  sur- 
face is  generally  provided  with  knobs  at  terminal  points, 
and  apparatus  for  collecting  or  discharging  electricity — 
such  as  lightning-rods — is  furnished  with  sharp  points.  To 
this  we  shall  again  refer  in  §  250. 

247.  .Induction.  —  The  experiments  described  in  §  241 
teach  us  that  a  body  may  be  electrified  by  friction  and  by 
contact  with  an  electrified  body.  Besides  these  two  ways, 
however,  bodies  in  their  usual  state  may  be  electrified  by 
influence  without  contact :  this  is  known  as  induction. 


Fig. 302. 


Fig.  302  will  aid  in  illustrating  this  subject.    JR,  represents  a 
metallic  sphere  insulated  by  a  glass  support,  and  charged 


362  NATURAL   PHILOSOPHY. 

with  positive  (+)  electricity;  a  b  represents  a  metallic  cylin- 
der insulated  in  the  same  manner,  to  which  pith  balls  are 
attached  by  silken  threads.  Now,  if  R  be  placed  near  a  b, 
but  not  near  enough  for  the  electric  spark  to  pass  from  one 
to  the  other,  it  will  destroy  the  equilibrium  of  the  two 
electricities  in  a  b — the  negative  electricity  being  accumu- 
lated at  the  end  near  the  sphere  R,  and  the  positive  at  the 
remote  end.  This  is  because  the  positive  electricity  in  R 
repels  its  like  in  a  b,  and  attracts  the  unlike  fluid.  'The  two 
pith  balls  at  the  positive  end  repel  each  other  because  they 
are  charged  with  the  same  electricity,  and  the  balls  at  the 
negative  end  are  mutually  repellent  for  the  same  reason. 
But  balls  hung  from  the  middle  would  not  be  affected,  be- 
cause they  are  on  middle  ground  between  the  two  electrici- 
ties. There  is  no  communication  of  electricity  from  R  to 
a  b,  but  only  an  influence  upon  the  quiescent  balanced  elec- 
tricities of  a  b.  Accordingly,  if  the  surplus  electricity  of 
R  be  discharged  by  putting  the  hand  or  any  good  conduct- 
or upon  it,  the  influence  will  cease,  the  equilibrium  in  a  b 
will  be  restored,  and  the  pith  balls  will  all  hang  straight 
down.  The  same  effect  will  be  produced  if  R  be  with- 
drawn to  a  distance  from  a  b,  and  the  influence  will  be  re- 
newed if  R  be  brought  near  again. 

If  instead  of  one  cylinder  we  should  use  two  placed  in 
contact,  the  negative  electricity  would  accumulate  in  one 
and  the  positive  in  the  other ;  and  if  the  two  cylinders  be 
suddenly  separated,  one  will  be  found  charged  with  nega- 
tive and  the  other  with  positive  electricity.  If  a  pane  of 
glass  be  held  between  R  and  a  b  (Fig.  302),  the  induction 
ensues  notwithstanding  the  intervening  insulator.  This  is 
additional  proof  that  the  electricity  does  not  actually  pass 
from  one  body  to  the  other. 

Electroscope. — The  pith  balls  serve  as  tests  of  the  presence  of  electricity ; 
but  a  far  more  delicate  means  is  supplied  by  the  simple  instrument  known 


ELECTRICITY. 


363 


as  the  gold-leaf  electroscope.  As  shown  in 
Fig.  303,  it  consists  of  an  insulated  glass 
vessel  into  which  a  metallic  rod  is  fastened, 
terminating  at  the  exterior  portion  in  a 
knob,  and  having  two  pieces  of  thin  gold- 
leaf  attached  to  the  end  within.  A  band  of 
tinfoil  is  glued  to  the  outside  of  the  glass 
sphere.  When  an  electrified  body  is  brought 
near  the  knob  of  the  electroscope,  the  gold 
leaves  fly  apart,  owing  to  induced  electricity. 
The  amount  of  divergence  is,  to  a  certain 
extent,  a  measure  of  the  force  of  the  electric 
charge.  When  used  for  this  purpose  the  in- 
strument is  called  an  electrometer  (§  248). 
Moreover,  when  a  body  having  like  elec- 
tricity is  brought  near  the  electrified  leaves, 
the  divergence  increases;  and  if  a  body 
charged  with  electricity  of  a  contrary  character,  the  leaves  fall  together. 
Hence  the  electroscope  may  be  used  to  determine  the  kind  of  electricity 
excited  in  any  substance. 

248.  The  Electrical  Machine. — You  are  now  prepared  to 
understand  the  operation  of  the  common  frictional  electri- 
cal machine.  There  are  two  kinds — the  plate  and  the  cy- 
lindrical ;  but  they  are 
both  constructed  on  es- 
sentially the  same  prin- 
ciple. Fig.  304  repre- 
sents a  plate  machine. 
P  is-  a  plate  of  glass 
which  can  be  revolved 
by  means  of  the  crank, 
M;  K  and  K  are  rub- 
bers of  silk,  the  pressure 
of  which  is  regulated 
by  screws.  These  rub- 
Fig.  304.  hers  are  connected  by 


364  NATURAL  PHILOSOPHY. 

the  support,  m,  and  the  chain,  T, with  the  floor;  or,  in  other 
words,  with  the  earth.  C  and  C'  are  the  so-called  prime 
conductors — hollow  cylinders — of  brass,  insulated  by  the 
pillars  of  glass,  VVVV.  These  conductors  terminate  at 
the  ends  next  to  the  glass  plate  in  rods  bearing  sharp 
points,  F  and  F',  which  serve  to  collect  the  electricity  ex- 
cited by  friction  on  the  glass  plate.  Certain  portions  of 
this  plate  are  covered  with  envelopes  of  silk,  which,  being 
non-conductors,  prevent  the  electricity  on  the  glass  from 
being  lost  in  the  air,  and  also  serve  to  keep  the  plate  free 
from  dust.  The  rubbers  are  cushions  of  horse-hair  covered 
with  leather,  and  coated  with  a  mixture  of  1  part  of  tin, 
1  part  of  zinc,  2  parts  of  mercury,  and  sufficient  lard — this 
amalgam  being  found  very  effectual  in  exciting  electricity. 

The  operation  of  the  machine  depends,  first,  on  the  devel- 
opment of  electricity  by  friction,  and,  secondly,  on  induction. 
When  the  glass  plate  is  revolved,  the  friction  of  the  rubbers 
causes  positive  electricity  to  collect  upon  the  glass  and  neg- 
ative upon  the  rubbers.  The  former  acts  by  induction  on  the 
prime  conductor,  and  the  latter  is  carried  off  by  the  chain 
to  the  earth.  Thus  the  conductor  loses  its  negative  electric- 
ity, and  gives  up  positive  electricity  in  sparks  to  any  other 
conductor  held  near  it.  No  actual  transfer  of  electricity 
from  the  plate  to  the  conductor  takes  place ;  there  is  mere- 
ly a  change  in  the  equilibrium  of  the  two  electricities. 

The  intensity  of  the  electricity  on  the  prime  conductor 
may  be  roughly  determined  by  means  of  the  small  electrom- 
eter B  (Fig.  304),  consisting  of  a  pith  ball  hanging  from  an 
upright  rod.  As  with  the  electroscope,  the  greater  the  in- 
tensity of  the  electricity,  the  farther  will  the  pith  ball  di- 
verge from  the  perpendicular. 

The  cylinder  machine  is  much  older  than  the  plate  machine 
just  described.  The  first  electrical  machine  was  construct- 
ed about  1650,  by  Otto  von  Gucricke,  the  inventor  of  the 


ELECTRICITY. 


365 


air-pump.  A  common  form  of 
the  cylinder  machine  is  shown 
in  Fig.  305.  The  glass  cylin- 
der, a  a,  can  be  turned  rap- 
idly by  the  multiplying-wheel, 
b  b.  At  c  is  a  piece  of  silk,  zj 
and  on  the  rear  part  of  the 
cylinder  is  the  rubber.  At  d 
is  the  prime  conductor,  insu- 
lated by  the  glass  support,  e.  Its  operation  is  precisely 
similar  to  that  of  the  plate  machine. 

249.  Experiments  with  the  Electrical  Machine. — The  first 
thing  noticeable  about  an  electrical  machine,  when  in  full 
operation,  is  the  succession  of  vivid  sparks  which  leap 
with  a  crackling  sound  from  the  rubber  to  the  prime  con- 
ductor. If  any  conductor  be  brought  near  the  end  of  the 
prime  conductor,  the  electricity  darts  to  it  at  intervals  with 
a  loud  snap.  These  sparks  passing  to  the  hand  of  a  person 
standing  by  produce  a  keen  pricking  sensation,  or  perhaps 
a  painful  numbness  along  the  arm. 

The  Insulating- Stool.  —  If  a  person  stand  on  a  wooden 

stool    supported    by   glass    legs 

(Fig.  306),    and 

hold  in  his  hand 

a  chain  connect- 
ed with  the  prime 

conductor,  he  will 

become  highly 

charged  with  electricity.  His  hair  will 
stand  up  on  his  head,  and  he  will  be  able 
to  give  electric  shocks  to  other  persons 
from  any  part  of  his  body.  Small  comic 
figures  charged  with  electricity  present  the 
appearance  shown  in  Fig,  307. 

Q 


Fig.  307. 


366  NATURAL   PHILOSOPHY. 

Other  Experiments. — Let  a  metallic  plate,  a,  Fig.  308,  be  sus- 
pended by  a  chain  to  the  prime  conductor,  and  another  plate,  i, 
be  supported  upon  a  conducting  stand.  If  figures  of  paper  or 
pith  be  placed  between  these  plates,  as  the  machine  is  worked, 
they  will  move  about  briskly  between  the  plates,  being  alternate- 
ly attracted  and  repelled  by  the  communication  of  the  electricity. 

The  experiment  represented  in  Fig.  309 
is  a  very  beautiful  one.  Let  a  b  be  a  braas 
rod  with  an  arch,  <?,  by  which  it  can  be 
suspended  from  the  end  of  the  prime  conductor. 
To  this  rod  are  suspended  three  bells,  the  two  outer 
ones  by  chains,  and  the  middle  one  by  a  silk  thread, 
c/; . also  two  clappers,  d  and  e,  by  silk  threads.  The  Fi°' 309' 

middle  bell  has  a  chain,/,  connecting  it  with  the  table — that  is,  with  the 
earth.  The  operation  of  the  apparatus  is  as  follows  :  As  soon  as  the  outer 
bells  become  electrified,  they  attract  the  clappers;  these,  on  touching  the 
bells,  receive  a  portion  of  their  electricity,  and  are  repelled.  They  therefore 
strike  against  the  middle  bell,  to  which  they  impart  the  electricity  received 
from  the  outer  bells.  They  then  swing  back  again  in  the  same  state  that 
they  were  in  at  first,  and  are  attracted  again  by  the  outer  bells.  This 
goes  on  so  long  as  the  electricity  is  communicated. 

Paste  upon  a  slip  of  glass  a  continuous  line  of  tinfoil,  going  back  and 
forth  as  represented  in  Fig.  310,  and  con- 
nect a  ball,  G,  with  one  end  of  the  foil, 
i-^—^—fE-E:— :==^  JL  Then  make  the  word  LIGHT  upon  it  by 

cutting  out  with  a  sharp  knife  little  por-- 
Fig.  310. 

tions   of  the  foil.       Placing  your   finger 

on  one  end  of  the  line  of  foil  at  a,  present  the  ball  G  to  the  prime  con- 
ductor, and  the  electric  fluid  will  run  along  the  whole  length  of  the  line 
from  G  to  a.  In  doing  this  the  letters  are  beautifully  illuminated,  a  spark 
being  produced  at  each  interruption  of  the  line.  So  rapid  is  the  pas- 
sage of  the  electricity  that  the  whole  appears  to  the  eye  simultaneously 
illuminated. 

250.  Electricity  Discharged  from  Points.  —  We  have  al- 
ready (§  246)  spoken  of  the  readiness  with  which  elec- 
tricity is  received  by  points.  It  is  discharged  from  them 
with  equal  readiness ;  so  that  if  a  metallic  point  be  at- 
tached to  the  prime  conductor,  the  electricity  will  be 


ELECTRICITY. 


367 


Fig.  311. 


carried  off  into  the  air  nearly  as 
fast  as  received;  arid  in  passing  off 
it  creates  a  current.  The  reaction 
of  the  air  upon  the  electrical  cur- 
rents can  be  very  prettily  exhibit- 
ed with  the  apparatus  represented 
in  Fig.  311,  which  consists  of  a  cap, 
resting  upon  the  point  of  a  rod, 
and  having  pointed  wires  branch- 
ing out  from  it  in  a  wheel-like  ar- 
rangement. You  observe  that  the 
points  are  all  bent  one  way.  If  this 
apparatus  be  set  upright  upon  the 
prime  conductor,  the  wheel  can  be 
made  to  revolve  rapidly  by  working 
the  electrical  machine.  In  the  same  way  that  the  reaction 
of  the  air  against  gases  issuing  from  a  rocket  makes  it  rise, 
reaction  against  the  electricity  issuing  from  these  points 
causes  the  circular  motion.  If  electricity 
be  discharged  from  a  point  in  a  darkened 
room,  it  appears  like  a  brush  of  light,  as 
represented  in  Fig.  312. 

251.  Leydeii-Jar.  —  The  Leyden-jar  is  a 
contrivance  for  storing  up  or  accumulating 
electricity.  It  is  so  named  because  the 
principle  upon  which  it  was  constructed 
Fig.  3i2.  was  discovered  in  Ley  den,  Holland.  An 
experimenter  endeavored  to  charge  with  electricity  a  vial 
of  water:  after  passing  many  sparks  into  it  through  a 
brass  rod  placed  in  the  vial,  he  took  hold  of  the  knob 
and  received  a  very  severe  shock.  Subsequent  experi- 
ments showed  that  the  charge  was  not  in  the  water,  and 
that  a  metallic  coating  within  and  without  the  glass  pro- 
duced better  effects.  A  common  form  of  the  Leyden-jar  is 


363 


NATURAL   PHILOSOPHY. 


A 


shown  in  Fig.  313.  It  consists  of  a 
glass  jar,  B,  coated  within  and  without 
to  near  the  top  with  tinfoil,  and  hav- 
ing a  metallic  rod  passing  through 
the  cork,  with  one  end  touching  the 
inner  coating,  and  the  other  surmount- 
ed by  a  brass  ball  or  knob,  A.  The  jar 
is  charged  by  holding  the  knob  near 
the  prime  conductor  of  an  electrical 
machine  in  operation.  The  electricity 
passes  by  the  metallic  rod  to  the  in- 
side coating  of  the  jar,  and  accumu- 
lates there.  This  is  positive  electricity.  In  the  meantime 
negative  electricity  accumulates  on  the  outside  coating,  in- 
duced by  the  positive  electricity  within.  The  positive  and 
negative  electricities  are  prevented  from  uniting  by  the 
non-conducting  glass  between  the  two  metallic  surfaces. 
If  a  slip  of  tinfoil  were  made  to  connect  the  inside  foil  with 
the  outer,  there  would  be  no  accumulation  of  electricity  on 
the  inside,  for  as  fast  as  it  passed  from  the  prime  conductor 
to  the  inside  it  would  pass  out  over  the  bridge  of  foil  to 
the  outside,  and  down  your  arm  and  body  to  the  earth. 

If  there  were  no  communication  of  the  outside  with  the  earth,  the  jar 
would  not  be  charged.  No  electricity  would  pass  to  it,  because  the  posi- 
vive  electricity  which  is  on  the  outside  cannot  be  driven  off,  and  no  nega- 
tive electricity  can  be  received.  To  make  this  plain,  suppose  that  the  jar 
o,  Fig.  314,  be  suspended  to  the  prime  conductor, 
b.  By  this  arrangement  the  inside  tinfoil  is  con- 
nected with  the  source  of  positive  electricity  and 
the  outside  is  insulated.  No  electricity  can  pass 
from  it  or  to  it.  It  has  both  positive  and  negative 
electricity,  but  they  are  in  equilibrium.  If  there 
were  a  preponderance  of  negative  electricity  there, 
it  would  attract  positive  electricity  to  it  as  near  as 
possible,  and  the  latter  would  enter  the  jar  from 
the  conductor.  But  there  is  no  such  preponderance,  and  although  n  littla 


Fig.  314. 


ELECTRICITY.  369 

may  enter — a  spark  or  two — there  will  not  be  enough  to  charge  the  jar 
sensibly.  But  bring  now  another  jar,  c,  near  to  the  outside  coating  of  a,  or 
connect  the  outer  coating  with  the  ground  by  means  of  a  chain,  and  there 
is  a  movement  at  once  in  the  electricities.  The  positive  electricity  has  a 
chance  now  to  pass  off  from  the  outside  of  a  to  the  inside  of  c,  leaving 
therefore  a  preponderance  of  negative  electricity  on  the  outside  of  the  jar. 
(See  §  218.) 

252.  Discharge  of  the  Leyden-Jar. — The  jar  may  be  dis- 
charged by  making  a  communication  between  the  inside 
and  outside  by  means  of  any  conductor.  This  may  be  done 
with  the  discharging -rod,  Fig.  315.  This  consists  of  two 
slender  metallic  rods,  with  brass  knobs  at  their 
ends,  and  jointed  at  «,  so  that  the  knobs  can  be 
separated  to  different  distances.  The  handle  is 
made  of  glass,  so  that  none  of  the  electricity 
passing  through  the  rods  may  be  communicated 
to  the  hand.  In  discharging  the  jar  one  knob  is 
placed  upon  the  outside  foil,  and  the  other  is 
brought  near  to  the  knob  of  the  jar.  Discharge 
follows,  electrical  equilibrium  is  restored,  and  a 
bright  flash  is  produced,  going  from  the  knob  of 
the  jar  to  that  of  the  discharging-rod,  and  this 
is  accompanied  with  a  report.  You  can  yourself  be  the 
conductor  to  discharge  the  jar.  If  you  place  one  hand 
upon  the  outside  of  the  jar,  and  bring  the  other  near  its 
knob,  the  fluids  will  meet  in  you  as  they  do  in  the  dis- 
charging-rod, and  a  shock  will  be  experienced  in  proportion 
to  the  amount  of  electricity  in  the  jar.  Any  number  of 
persons  can  simultaneously  receive  the  same  shock.  To  do 
this  they  must  join  hands,  and  the  person  at  one  end  of  the 
row  must  touch  the  knob  of  the  jar  while  the  person  at 
the  other  end  has  his  hand  upon  the  outside. 

You  may  touch  either  the  knob  of  the  jar  or  the 
outside  coating  separately,  and  the  power  in  it  remains 
quiet;  but  the  moment  you  touch  both  it  bursts  forth, 


370 


NATURAL   PHILOSOPHY. 


because  a  bridge  is  made  over  which  the  electricity  can 
pass. 

In  a  dry  air  the  charge  in  the  jar  can  be  retained  for  some  time,  the 
communication  between  the  two  electric  fluids  being  very  slow  through  the 
medium  of  air.  It  is  otherwise  when  there  is  much  moisture  in  the  air,  for 
water  is  a  good  conductor.  For  this  reason,  if  you  let  the  moisture  from 
your  breath  come  upon  the  jar  between  the  outside  coating  and  the  rod, 
the  jar  will  be  discharged  soon,  though  imperceptibly,  the  moisture  making 
a  medium  of  communication  between  the  inner  and  outer  electricities. 

Electrical  Battery.  —  By  combining  together  a  num- 
ber of  jars,  having  the  insides  all  connected  together,  as 

seen  in  Fig.  316,  with 
metallic  rods,  and  the 
outsides  connected 
together  in  a  similar 
manner,  we  form  what 
is  termed  an  electric- 
al battery.  With  such 
an  arrangement  we 
can  accumulate  a 
large  amount  of  elec- 
tricity, which  can  be 
discharged  in  essen- 
tially the  same  way 
as  in  the  case  of  the  single  jar.  Persons  experimenting 
with  a  large  battery  are  obliged  to  exercise  care  in  avoid- 
ing an  accidental  discharge  of  the  whole  electricity  through 
the  body,  the  effects  of  which  are  very  uncomfortable,  if  not 
positively  injurious. 

253.  Light  of  Electricity. — The  light  produced  by  elec- 
tricity is  not  occasioned  by  anything  like  combustion.  It 
depends  obviously  upon  the  resistance  offered  to  its  pas- 
sage. Thus,  when  the  electric  fluid  passes  through  air 
from  the  prime  conductor  to  the  knob  of  the  Leyden-jar, 


Fig.  310. 


ELECTRICITY.  371 

it  causes  a  flash  of  light ;  but  when  it  arrives  at  the  knob, 
the  flash  ceases.  What  is  the  reason  of  the  difference  ?  In 
both  cases  it  meets  the  resistance  of  the  air,  for  when  it 
reaches  the  knob  it  passes  over  the  surface  of  the  knob  and 
rod ;  but  in  the  latter  case  it  is  so  diffused  over  the  metal- 
lic surface  that  it  meets  with  much  less  resistance  from  the 
air.  By  experiments  with  the  air-pump  it  is  found  that  the 
denser  the  air,  the  more  vivid  the  spark;  and  if  electricity 
be  passed  through  a  glass  vessel  exhausted  of  air,  it  forms 
streams  of  light  resembling  the  aurora  borealis,  which  are 
so  strikingly  in  contrast  with  the  vivid  flashes  of  the  light- 
ning. In  the  experiment,  §  249,  in  which  the  word  LIGHT  is 
made  by  the  passing  electricity,  we  have  a  striking  illus- 
tration of  the  production  of  the  spark  by  the  resistance  of 
the  air.  If  the  foil  were  one  continuous  surface,  the  elec- 
tricity would  be  diffused  over  it  without  giving  any  light. 
It  is  only  where  the  electric  fluid  leaps  through  the  air 
from  one  portion  of  foil  to  another  that  light  is  seen. 

254.  Other  Phenomena  of  Electricity. — The  report  of  elec- 
tricity is  a  sort  of  crack  or  snap  from  the  sudden  condensa- 
tion of  the  air  by  the  rapid  passage  of  the  fluid.  The  roll- 
ing of  thunder  is  occasioned  by  the  reverberation  of  the 
first  sound  among  the  clouds.  The  nearer  the  flash,  the 
more  like  a  crack  is  the  first  sound  which  reaches  our  ears. 

Mechanical  Effects. — When  any  great  amount  of  elec- 
tricity meets  in  its  passage  with  any  imperfect  conductor, 
it  does  much  violence  to  it.  Thus  it  rends  wood,  scatters 
water,  breaks  glass,  etc.  Various  experiments  illustrate 
the  manner  in  which  mechanical  effects  result  from  elec- 
tricity. Thus,  if  it  be  passed  through  a  card  or  several 
leaves  closely  pressed  together,  a  burr  forms  on  each  side 
of  such  a  character  as  to  show  that  two  forces  moving  in 
opposite  directions  have  forced  their  passage. 

Production  of  Heat. — Electricity  always  produces  in  its 


372  NATURAL  PHILOSOPHY. 

passage  a  certain  amount  of  heat,  probably  by  its  me* 
chanical  effect.  When  diffused  over  a  large  conducting 
surface,  the  heat  is  not  noticeable  ;  but  if  confined  to 
the  surface  of  a  small  wire,  the  heat  may  be  sufficient 
to  melt  or  even  burn  it.  Various  effects  can  be  pro- 
duced by  the  heat  thus  caused  by  the  passage  of  elec- 
tricity. Gunpowder  may  be  exploded  by  it.  Alcohol 
and  ether  may  be  readily  ignited  by  it,  especially  the 
latter.  Gas  can  sometimes  be  lighted  by  touching  with 
the  finger  an  opened  burner  after  walking  across  the  room 
two  or  three  times  briskly,  rubbing  the  feet  upon  a  thick 
carpet. 

The  convertibility  of  the  three  forces — heat,  light,  and 
electricity  —  is  illustrated  by  the  phenomena  above  de* 
scribed;  they  are  all  modes  of  motion.  (See  §  165.) 

25.5.  Franklin's  Discovery.  —  It  had  very  early  been  conjectured 
that  the  electricity  produced  by  the  electrical  machine  is  identical  with 
lightning;  but  it  was  reserved  for  our  countryman  Benjamin  Franklin  to 
prove  the  fact.  He  thought  of  making  use  in  his  investigations  of  a  tall 
spire  which  was  erecting  in  Philadelphia  (in  1752),  but  before  it  was  com- 
pleted the  sight  of  a  boy's  kite  in  the  air  suggested  to  him  another  plan. 
He  made  a  kite  by  stretching  a  silk  handkerchief  over  a  frame,  and  raised 
it  during  an  approaching  thunder-shower,  his  only  companion  being  his  son. 
Having  raised  the  kite,  he  attached  to  the  end  of  the  hempen  string  a  key, 
and  also  a  silk  ribbon,  by  which  he  insulated  his  apparatus,  as  seen  in  Fig. 
317.  He  then  watched  with  much  anxiety  the  result.  A  cloud  arose, 
which  he  supposed,  from  its  appearance,  was  well  charged  with  electricity, 
and  yet  no  effect  was  seen.  Franklin  began  to  despair ;  but  at  length  he 
saw  some  loose  fibres  of  the  hempen  string  bristling  up,  and,  applying  his 
knuckle  to  the  key,  received  just  such  a  spark  as  he  had  often  received 
from  the  conductor  of  an  electrical  machine.  The  discovery  was  accom- 
plished, and  Franklin  was  at  once  overcome  with  emotion  at  the  thought 
of  the  immortality  which  it  would  give  his  name.  The  fame  of  the  dis- 
covery, made  in  a  manner  so  simple  and  yet  so  original,  spread  everywhere, 
and  prompted  to  many  experiments  by  other  philosophers.  One,  Professor 
Richman,  of  St.  Petersburg,  fell  a  victim  to  his  investigations.  While 
attending  a  meeting  of  the  Academy  of  Sciences  he  heard  the  sound  of 


ELECTRICITY. 


373 


Fig.  317. 

distant  thunder,  and  hastened  home  to  make  some  observations  with  an 
apparatus  which  he  had  erected.  While  doing  this  a  charge  of  elec- 
tricity flashed  from  the  conducting- rod,  and,  piercing  his  head,  killed 
him  instantly.  His  assistant,  who  stood  near,  was  struck  down,  and  re- 
mained senseless  for  some  time,  and  the  door  of  the  room  was  torn  from 
its  hinges. 

256.  Lightning-Rods. — The  discovery  of  Franklin  led  to  the  custom 
of  attaching  lightning-rods  to  buildings.  The  object  of  a  lightning-rod  is 
to  conduct  any  electricity  in  a  cloud  that  may  come  over  the  building 
down  into  the  ground.  For  this  purpose  the  rod  should  terminate  in 
the  air  in  points,  as  these  readily  receive  the  electric  fluid  (§  250).  The 
rod  should  be  separated  from  the  house  by  glass  supports,  and  it  should 
pass  so  far  into  the  ground  as  to  terminate  in  moist  earth.  The  points 
should  be  gilded  in  order  to  be  preserved  from  corrosion-,  or  they  may 
be  made  of  silver  or  platinum.  Lightning-rods  are  undoubtedly  often  of 

Q2 


374  NATURAL  PHILOSOPHY. 

service  when  there  is  no  obvious  passage  of  the  lightning  down  them,  by 
quietly  and  continuously  receiving  electricity  upon  their  points,  and  pass- 
ing it  down  into  the  earth. 


QUESTIONS. 

241.  What  is  the  origin  of  the  term  electricity  ?  What  is  nature  ?  Name 
the  simple  apparatus  necessary  to  illustrate  experimentally  the  fundamental 
facts  of  electricity.  Describe  the  experiments  with  the  glass  rod  and  silk 
handkerchief.  How  is  attraction  shown?  How  repulsion?  What  is 
meant  by  an  electrified  body  ?  Describe  the  experiment  with  the  poker, 
wine-glasses,  etc.  Describe  the  remaining  experiments. — 242.  What  the- 
ories have  been  proposed  to  account  for  electrical  phenomena?  Give 
Franklin's  theory.  State  the  two-fluid  theory.  What  is  meant  by  resinous 
and  vitreous  electricity  ?  What  by  negative  and  positive  ?  What  is  the 
modern  theory? — 243.  What  substances  generate  positive  electricity? 
What  ones  negative?  State  the  law  as  to  like  and  unlike  electricities. — 
244.  Give  in  full  what  is  said  of  the  active  presence  of  electricity. — 245. 
What  is  meant  by  conductors  and  non-conductors  ?  Name  the  best.  What 
are  insulators,  and  why  so  called  ?  What  is  said  of  electrics  and  non- elec- 
trics? Is  this  a  proper  means  of  classification  ? — 246.  Describe  an  experi- 
ment showing  the  manner  in  which  electricity  is  disposed  in  a  body.  De- 
scribe Faraday's  experiment.  How  does  electricity  distribute  itself  over  a 
sphere  ?  How  over  an  oblong  body  ?  What  is  the  advantage  of  knobs  to 
apparatus?  What  of  points? — 247.  What  is  meant  by  induction  ?  Explain 
in  full  the  manner  in  which  an  electrified  body  induces  electricity  in  an- 
other. What  is  an  electroscope  ?  Describe  one  form,  and  show  how  it 
may  be  used. — 248.  Describe  the  plate  electrical  machine,  stating  the  use 
of  each  part.  How  is  electricity  generated  by  it  ?  Describe  the  cylinder 
machine. — 249.  Describe  some  experiments  with  the  electrical  machine. 
What  is  an  insulating  stool?  Explain  the  action  of  the  electric  bells.  De- 
scribe the  experiment  showing  the  light  of  electricity. — 250.  What  is  said 
of  the  escape  of  electricity  from  points  ? — 251.  Describe  the  Leyden-jar. 
How  was  it  discovered  ?  What  is  its  use  ?  Show  why  an  insulated  Leyden- 
jar  cannot  be  charged  with  electricity. — 252.  Explain  the  use  of  the  dis- 
charging-rod.  How  can  a  large  number  of  persons  take  a  shock  simul- 
taneously ?  Explain  the  effect  of  moisture  upon  the  charged  jar.  What  is 
the  electrical  battery  ? — 253.  What  is  said  of  the  light  produced  by  elec- 
tricity ? — 254.  To  what  is  the  report  of  electricity  owing  ?  What  is  said 


GALVANISM.  375 

of  mechanical  injures  caused  by  electricity?  What  of  the  heat  caused  by 
it  ?  What  effects  may  be  produced  by  this  heat  ? — 255.  What  was  the  dis- 
covery of  Franklin,  and  how  did  be  make  it  ?  Relate  the  accident  which 
occurred  at  St.  Petersburg. — 256.  What  is  said  of  lightning-rods  ?  How. 
should  tliey  be  constructed  ? 


CHAPTER  XIX. 

GALVANISM. 

257.  Galvanic  or  Voltaic  Electricity.  —  In  the  preceding 
chapter  you  learned  something  of  the  effects  of  electricity 
generated  by  friction.  There  is  another  form  of  electricity, 
commonly  called  Galvanic  or  Voltaic  Electricity,  produced 
by  chemical  action.  Frictional  electricity,  as  we  shall  now 
call  that  form  previously  studied,  was  recognized  as  a  pecul- 
iar force  more  than  six  hundred  years  before  Christ;  but 
galvanism  was  not  discovered  until  the  end  of  the  last  cen- 
tury. The  history  of  its  discovery  is  interesting.  The  dawn 
of  galvanism  is  found  in  an  observation  made  by  Sulzer,  a 
citizen  of  Berlin,  in  1767.  He  states  that  if  a  piece  of  zinc  be 
placed  under  the  tongue,  and  a  piece  of  silver  upon  it,  when 
the  two  metals  are  brought  in  contact  a  metallic  taste  is 
perceived,  and  a  shock  felt  by  the  tongue.  Sulzer  attrib- 
uted the  effect  to  some  vibratory  motion  occasioned  by  the 
contact  of  the  metals,  and,  satisfied  with  this  fanciful  ex- 
planation, pursued  the  inquiry  no  further.  The  statement 
excited  but  little  notice  until  other  facts  of  a  similar  char- 
acter were  brought  out  in  1790  by  Galvani,  professor  of 
anatomy  at  Bologna.  He  observed  that  the  legs  of  some 
frogs,  which  had  been  obtained  for  his  invalid  wife,  were 
convulsed  by  touching  the  nerves  with  a  knife  when  near 
an  excited  electrical  machine.  In  contrast  with  the  ex- 


376 


NATURAL  PHILOSOPHY. 


ample  of  Sulzer  he  was  led  to  pursue  the»matter  further. 
He  found  that  similar  muscular  contractions  took  place 
even  when  no  electricity  was  communicated  from  the  ma- 
chine. Having  suspended  some  partially  dissected  frogs 
by  copper  hooks  from  the  iron  bar  of  the  balcony  of  his 
window,  he  observed  that  whenever,  from  accidental  causes, 
the  muscles  of  the  frogs'  legs  came  into  contact  with  the 
iron  bar,  the  limbs  were  convulsed  much  in  the  same  way 
as  with  frictional  electricity.  In  consequence  of  this  ob- 
servation Galvani  made  the  following  experiment,  which 
has  since  become  famous.  He  cut  a  frog  directly  in  two, 
skinned  its  hind-legs,  and  then  arranged  pieces  of  zinc  and 
copper,  as  shown  in  Fig.  318.  The  zinc  is  in  contact  with 


Fig.  313. 


the  nerves  which  connect  the  legs  and  the  spine,  and  the 
copper  is  in  contact  with  the  muscles  of  the  legs.  Now, 
whenever  the  two  ends  of  the  metal  strips  are  made  to 
touch  each  other,  the  muscles  are  strongly  convulsed,  and 


GALVANISM.  377 

each  leg  is  thrown  up  into  the  position  shown  by  the 
dotted  lines.  Galvani  erroneously  supposed  this  to  be  an 
exhibition  of  animal  electricity,  regarding  the  muscles  as 
a  kind  of  Leyden-jar,  the  nerve  being  the  medium  of  com- 
munication with  the  inside. 

258.  Volta's  Pile. — The  observations  of  Galvani  awrakened 
much  interest  in  all  scientific  minds,  and,  of  course,  there 
was  much  inquiry^  observation,  and  experiment.  Professor 
Volta,  ofPavia,  went  a  step  further  than  Galvani  towards 
the  true  explanation,  in  referring  the  effects  to  the  con- 
tact of  dissimilar  metals;  and  he  was  led  by  this  view  of 
the  subject  to  construct  his  pile,  or  battery — called  after 
him  the  voltaic  pile — the  object  of  which  is  to  produce  a 
much  greater  amount  of  electricity  than  can  be  obtained 
by  the  contact  of  only  two  pieces  of  metal.  The  pile  is 
made  of  circular  pieces  of  copper,  zinc, 
and  cloth,  the  cloth  being  moistened  c 
with  salt-water  or  a  weak  acid.  They  Z 
are  arranged  as  represented  in  Fig.  319. 
First  a  disk  of  copper  is  laid  down,  then  C 
upon  this  one  of  zinc,  then  one  of  cloth, 
and  so  on  in  the  same  order,  the  top  of 
the  pile  ending  in  a  plate  of  zinc.  If  yon 
touch  one  end  of  the  pile  with  a  moist- 
ened finger,  and  the  other  end  with  a  finger  of  the  other 
hand,  you  will  feel  a  shock  somewhat  like  that  from  a  Ley- 
den-jar.  The  communication  between'the  two  ends  of  the 
pile  may  be  made  by  wires,  as  in  the  figure.  That  con- 
nected with  the  zinc  disk  furnishes  positive  electricity, 
and  that  attached  to  the  copper  disk,  negative.  When  the 
ends  of  these  wires  are  joined,  sparks  of  galvanic  electrici- 
ty pass  between  them,  the  intensity  being  in  proportion  to 
the  number  of  couples,  as  the  pairs  of  zinc  and  copper  are 
called.  This  arrangement  is  essentially  the  same  as  in  the 


378  NATURAL   PHILOSOPHY. 

experiments  of  Sulzer  and  of  Galvani,  the  cloth  taking  the 
place  of  the  tongue  in  the  one,  and  of  the  frog's  flesh  in  the 
other — moisture  being  present  in  each  case. 

Volta's  explanation  of  this  development  of  electricity — 
the  so-called  "  contact  theory  " — was  long  received  as  the 
true  one;  but  it  has  gradually  been  abandoned,  and  the 
production  of  electricity  is  now  believed  to  depend  upon 
chemical  action. 

To  distinguish  this  form  of  electrical  force  from  frictionnl  electricity,  it 
is  often  called  Galvanism,  after  its  illustrious  discoverer.  It  has  also  been 
called  Voltaism,  after  Volta,  who  added  greatly  to  the  discoveries  concern- 
ing this  wonderful  agency.  Another  term  often  applied  to  it  is  dynamical 
electricity  as  opposed  to  statical  electricity,  a  name  given  to  frictional  elec- 
tricity. Statical  signifies  stationary,  because  the  electricity  accumulated 
by  friction  or  other  means  remains  stationary  or  at  rest  until  some  way  be 
provided  for  its  escape.  It  exerts  no  force  until  this  is  done.  Dynamical 
electricity,  on  the  other  hand,  is  in  motion  or  action  at  the  instant  of  its 
production,  the  word  dynamical  being  derived  from  a  Greek  word  meaning 
force  or  power. 

259.  Voltaic  Circle. — If  a  bar  or  slip  of  zinc,  Z,  Fig.  320, 
and  a  bar  of  copper,  C,  be  placed  in  a  mixture 
of  water  and  sulphuric  acid,  and  a  metallic  rod, 
a,  be  laid  across  the  top  of  them,  the  liquid  will 
become  turbid,  and  a  current  of  electricity  will 
flow  throiiffh  the  liquid  and  across  the  metallic 

Fig.  320.  .  .  ... 

rod  in  the  direction  indicated  by  the  arrows. 
If  the  rod  be  cautiously  lifted  from  either  the  zinc  or  the 
copper,  a  spark  will  pass  at  the  moment  that  the  contact  is 
broken,  which  will  be  visible  if  the  room  be  darkened.  If 
the  rod  be  not  placed  upon  the  bars,  the  electrical  action 
excited  in  the  first  moment  soon  ceases,  and  there  will  be 
no  current,  for  the  circuit  must  be  perfect  to  obtain  this 
result.  If  a  rod  of  glass  be  used  instead  of  the  metallic 
rod,  the  circuit  will  be  incomplete  —  glass  being  a  non- 
conductor of  the  electric  fluid  —  and  therefore  no  effect 


GALVANISM.  379 

will  be  produced.  The  simple  apparatus  rep- 
resented in  Fig.  320  contains  all  the  essentials 
of  what  is  called  the  voltaic  or  galvanic  circle. 
It  may  be  made  even  more  simple  than  this, 
as  shown  in  Fig.  321,  the  ends  of  the  slips  of 
zinc  and  copper  which  are  out  of  the  liquid 
being  made  to  touch.  Commonly  in  galvanic  experiments 
the  connection  between  the  two  metals  is 
made  by  metallic  wires,  as  seen  in  Fig.  322. 
Copper  wire  is  ordinarily  used.  Platinum 
is  often  employed,  because  in  experiments 
it  is  sometimes  necessary  to  introduce  the 
ends  of  the  wires  into  corrosive  liquids. 
The  arrangement  indicated  is  called  the 
simple  voltaic  circle.  The  compound  cir- 
cle will  be  described  shortly. 

The  electricity  produced  as  described  above  is  the  re- 
sult of  a  chemical  action  between  the  liquid  and  the 
zinc.  Through  the  agency  of  the  acid,  water  is  decom- 
posed into  its  constituents — oxygen  and  hydrogen,*  the 
oxygen  uniting  with  the  zinc  to  form  an  oxide  of  zinc,  and 
the  hydrogen  going  to  the  copper  and  appearing  there 
in  bubbles,  which  escape  into  the  air.  Just  as  fast  as  the 
oxide  of  zinc  is  formed  the  sulphuric  acid  combines  with 
it,  forming  sulphate  of  zinc,  which  remains  dissolved  in  the 
water.  This  is  quite  an  essential  part  of  the  operation,  for 
if  the  oxide  were  not  removed,  it  would  very  soon  coat 
over  the  zinc,  and  so  protect  it  from  the  liquid  as  to  stop 
the  chemical  action.  The  copper  acts  simply  as  a  conduct- 
or of  the  electricity  developed  by  the  oxidation  of  the  zinc. 
Although  the  electricity  cannot  be  produced  unless  this 


*  For  explanation  of  these  and  other  chemical  terms,  see  Part  II., 
Chemistry. 


380  NATURAL  PHILOSOPHY. 

chemical  action  take  place,  and  therefore  may  be  consid- 
ered the  result  of  it,  yet  the  chemical  action  will  not  oc- 
cur unless  the  circuit  be  complete.  The  zinc  and  the  cop- 
per may  stand  in  the  liquid,  but  everything  will  be  quiet 
until  the  two  metals  are  connected.  The  electrical  passage 
or  way  must  be  opened,  or  there  will  be  no  chemical  action 
to  produce  the  electricity. 

260.  Amalgamation  of  Zinc. — The  zinc  used  in  galvanic 
apparatus  is  usually  amalgamated — that  is,  combined  with 
mercury.     When  perfectly  pure  zinc  is  used,  the  liquid 
does  not  attack  it  until  the  electrical  current  is  established. 
But  since  this  metal,  as  usually  obtained,  is  impure,  the  liq- 
uid does  act  upon  it  even  before  the  zinc  and  copper  are 
connected,  and  therefore  there  is  a  useless  waste  of  the  zinc. 
There   is  waste  in  another  way.     The  particles  of  other 
metals  mingled  with  the  zinc  occasion  chemical  and  elec- 
trical action  when  the  acid  liquid  reaches  them.     There 
is,  therefore,  a    local    production    of  electricity   at   these 
points,  which  is  disposed   of  just  where   it  is   produced, 
and  does  not  pass  to  the  conducting  wires.     It  is  to  pre- 
vent  this   local  chemical   and   electrical   action   that   the 
zinc  is  amalgamated,  the  acid   liquid   having  little  or  no 
effect  upon  the  amalgamated  zinc  until  the  circuit  is  com- 
pleted. 

Sheathing  of  Vessels. — In  the  copper-zinc  battery  the  copper  is  not  at- 
tacked by  the  acid  liquid,  the  presence  of  the  oxidizable  zinc  acting  as  a 
protection.  Knowing  this,  Sir  Humphry  Davy  suggested  that  pieces  of 
zinc  connected  here  and  there  with  the  copper  sheathing  of  vessels  would 
prevent  it  being  corroded  by  the  sea-water.  The  experiment  was  tried 
and  was  successful ;  the  copper  remained  bright  and  clean.  At  the  same 
time,  however,  the  plan  was  abandoned,  because  the  uncorroded  copper  be- 
came covered  with  shell-fish  and  marine  plnnts,  the  accumulation  of  which 
seriously  impeded  the  progress  of  the  vessel  through  the  water. 

261.  Polarity  in  Galvanism.  —  Polarity  is  manifested  in 


GALVANISM. 


381 


galvanism,  as  indicated  by  Fig.  323.  Posi- 
tive electricity  "flows"  from  the  immersed 
end  of  the  zinc  marked  -f  (plus),  while  the 
immersed  end  of  the  copper  is  negative, 
marked  —  (minus).  The  ends  in  the  air  exhib- 
it polarity  the  reverse  of  those  in  the  liquid. 
The  terminal  wires  of  a  battery  are  called  the 
poles,  being  positive  or  negative,  according 
to  the  metals  with  which  they  are  connected.  The  terms 
"fluid"  and  "current"  are  very  generally  used,  but  their 
use  must  not  be  considered  as  implying  that  there  is  any 
real  fluid  flowing  in  a  current.  These  and  other  kindred 
terms  are  convenient  in  indicating  processes  and  results, 
and  are  used  solely  on  this  account,  and  not  as  denoting  a 
belief  in  the  existence  of  a  fluid.  Electricity  is  a  force  of 
matter,  but  the  nature  of  this  force  is  not  known.  That  it 
moves  in  circuits  and  manifests  polarity  is  obvious  to  every 
one;  and  hence  the  terms  fluid  and  current,  and  positive 
and  negative  electricities,  may  be  used  in  a  somewhat  fig- 
urative manner  without  the  violation  of  truth. 

The  terms  electro-positive  and  electro-negative  may  be 
here  defined.  We  explained  in  §  259  that  in  the  decom- 
position of  the  acid  liquid  the  oxygen  goes  to  the  zinc  and 
the  hydrogen  to  the  copper.  Now,  as  shown  in  Fig.  323, 
the  immersed  end  of  the  zinc  is  positive,  and  that  of  the 
copper  is  negative ;  and,  since  attraction  is  between  oppo- 
site electricities  (§  242),  the  oxygen,  which  is  attracted  by 
the  positive  zinc,  is  called  electro-negative;  while  the  hy- 
drogen, which  is  attracted  by  the  negative  copper,  is  called 
electro-positive.  And  all  substances  are  put  into  one  class 
or  the  other,  according  to  their  galvanic  affinities.  (See 
§  264.) 

The  polarity  of  galvanism  will  be  further  illustrated 
when  we  speak  of  the  effects  of  this  agent. 


382  NATURAL   PHILOSOPHY. 

262.  Galvanism  Produced  in  Various  Ways. — We   have 
thus  far   mentioned  only  one  way  of  producing  galvan- 
ism, but  the  means  of  producing  it  may  be  very  greatly 
varied.     Other  metals  can  be   substituted  for  copper  in 
the  voltaic  circle,  provided  they  are  less  oxidizable  than 
zinc.     Thus  zinc  and  lead  will  answer,  but  by  no  means  so 
well  as  zinc  and  copper.     Zinc  and  platinum  are  more  ac- 
tive in  producing  galvanism  than  zinc  and  copper,  though 
the  latter  combination  is  more  generally  used  because  cop- 
per is  so  much  cheaper  than  platinum.    Other  acids  besides 
sulphuric  can  be  used,  as  hydrochloric,  etc.     Other  sub- 
stances besides  metals  are  competent  to  produce  galvanic 
electricity.     Even  vegetable  and  animal  substances  have 
been  used  with  success,  showing  how  universal  galvanism 
is  as  an  agent  in  nature.     In  all  cases  the  positive  electric- 
ity comes  from  the  substance  most  readily  acted  upon  by 
the  fluid  used,  and  the  negative  from  the  other.     Though 
galvanic  electricity  is  ordinarily  produced  by  an  arrange- 
ment of  two  solids  in  a  fluid,  it  may  be  produced  by  a  solid 
with  two  fluids.    Thus,  when  a  plate  of  zinc  is  brought  into 
contact  with  salt-water  on  one  side,  and  hydrochloric  acid 
on  the  other,  a  current  of  electricity  will  flow  from  the  for- 
mer to  the  latter  liquid. 

263.  Volta's  "Crown  of  Cups." — Soon  after  Volta  invent- 
ed his  pile  he  made  the  first  voltaic  battery — known  as  the 
"  crown  of  cups."     It  is  essentially  the  same  as  the  pile, 
being  the  combination  of  a  number  of  simple  voltaic  circles, 
and  hence  sometimes  called  a  compound  voltaic  circle.     It 
is  represented  in  Fig.  324.     The  cups  contain  a  weak  acid 
or  a  saline  liquid,  and  in  each  is  placed  a  plate  of  zinc  and 
one  of  copper,  the  arrangement  being  such  that  the  cop- 
per plate  of  the  first  cup  is  connected  by  a  wire  with  the 
zinc  plate  of  the  second,  and  the  same  with  the  second 
and  third,  and  so  through  the  series,  however  long  it  may 


GALVANISM. 


383 


be.  The  positive  electricity  flows  in  each  cup  from  the  zinc 
to  the  copper,  then  onward  by  the  connecting  slip  of  metal 
to  the  zinc  of  the  next  cup,  and  so  on  until  it  passes  out  by 
the  wire  connected  with  the  copper  in  the  last  cup  to  meet 
the  electricity  coming  from  the  opposite  direction.  The 
course  of  the  positive  electricity  is  indicated  by  the  ar- 
rows. 

By  thus  uniting  a  number  of  single  cups  a  far  greater 
quantity  of  electricity  is  generated,  and  very  remarkable 
effects  can  be  produced.  A  modification  of  this  battery  is 
shown  in  Figs.  325  and  326.  It  consists  of  a  long  wooden 


Fig.  325. 


Fig.  L2G. 


trough  divided  into  narrow  cells  by  pairs  of  square  plates 
of  zinc  and  copper  soldered  together.  Into  these  cells  a 
saline  solution  is  poured.  Wires  connected  with  either  end 
serve  to  conduct  the  voltaic  current.  This  form  of  battery 
was  found  more  convenient  than  the  "  crown  of  cups,"  ou 
account  of  its  portability. 


384 


NATURAL   PHILOSOPHY. 


264.  Other  Batteries. — Batteries  have  been  varied  much, 
both  as  regards  the  material  and  the  arrangement.  Some 
of  these  forms  we  shall  briefly  describe.  You  learned  in 
§  262  that  electricity  may  be  generated  by  contact  of  a  solid 
with  two  fluids;  it  is  also  commonly  produced  by  a  pecul- 
iar arrangement  of  two  solids  and  two  fluids.  A  battery 
in  which  only  one  fluid  is  used  soon  loses  its  power,  or  runs 
down — as  it  may  be  expressed;  but  those  in  which  two 
fluids  are  used  are  more  reliable  and  constant,  as  well  as 
far  more  powerful.  One  of  these  forms,  known  as  DanielPs 
battery,  is  represented  in  Fig.  327.  The  external  glass 

cup  contains  a  cylinder 
of  copper,  K,  within 
which  is  another  cylin^ 
dor  of  unglazed  porce- 
lain, T,  which  latter 
contains  a  rod  or  cylin- 
der of  cast  zinc,  Z.  Di- 
lute sulphuric  acid 
poured  into  the  porous 
cell  acts  upon  the  zinc. 
Into  the  outer  glass  cup 
is  poured  a  strong  solu- 
tion of  sulphate  of  cop- 
per, some  of  the  solid 
crystals  being  added  to 
keep  the  solution  con- 
centrated. For  the  con- 
venience of  connection 
with  wires  or  with  other 
similar  batteries,  the  metallic  strips,  ra  and  p,  are  fastened 
to  the  zinc  and  the  copper  cylinders  respectively;  the 
screw,  s,  assists  in  joining  two  such  cups.  By  this  com- 
pound arrangement  the  porous  cell  prevents  the  passage 


GALVANISM.  385 

of  metallic  particles  between  the  copper  and  the  zinc ;  and 
the  deposition  of  copper,  resulting  from  the  decomposition 
of  the  sulphate,  keeps  the  copper  cylinder  clean  and  al- 
ways ready  for  action.  Daniel  1's  battery  generates  gal- 
vanism with  great  uniformity  and  constancy,  and  in  its 
varied  forms  has  been  much  used  for  telegraphic  purposes. 
By  combining  a  number  of  these  cups  by  the  aid  of  the  me- 
tallic strips  and  screws,  a  very  powerful  battery  is  ob- 
tained. The  copper  in  each  cup  is  connected  with  the  zinc 
of  the  next,  as  shown  in  Fig.  328.  The  negative  and  posi- 
tive poles  are  marked  n  and  p  respectively. 


Fig. 328. 

The  batteries  of  Grove  and  Bnnsen  are  very  important 
on  account  of  their  superior  power.  Bunsen's  cup  or  cell 
differs  from  Daniell's,  both  in  the  material  and  arrange- 
ment. The  copper  is  replaced  by  a  cylinder  of  coke  or  car- 
bon, and  the  sulphate  of  copper  solution  by  strong  nitric 
acid.  The  outer  cylinder  is  of  zinc ;  within  is  a  porous 
cell  containing  the  carbon  and  filled  with  nitric  acid.  The 
latter  serves  to  remove  the  hydrogen  liberated  at  the  car- 


386  NATURAL   PHILOSOPHY. 

bon,  or  positive,  end,  and  prevents  the  battery  from  losing 
its  power  so  rapidly  as  it  otherwise  would. 

Grove's  battery  is  similar  in  all  respects  to  Bmisen's,  ex- 
cept that  platinum  is  substituted  for  carbou ,'  and,  although 
this  metal  is  quite  expensive  at  the  outset,  so  great  is  the 
efficiency  that  it  is  economical  in  the  end. 

The  metals  used  in  these  batteries,  as  well  as  the  carbon, 
are  not  selected  for  the  purpose  at  hazard,  but  in  accord- 
ance with  a  well-defined  law — viz.,  the  greater  the  differ- 
ence in  the  oxidability  of  the  metals  taken,  the  stronger 
the  electric  current  obtained.  The  order  of  oxidability  of 
the  principal  metals  is  as  follows : 

1.  Zinc.  7.  Antimony. 

2.  Tin.  8.  Copper. 

3.  Lead.  9.  Silver. 

4.  Iron.  10.  Gold. 

5.  Nickel.  11.  Platinum. 

6.  Bismuth.  12.  Carbon  (a  non-metal). 

The  most  readily  oxidized  by  immersion  in  dilute  acid  is 
the  zinc;  the  least,  carbon.  Any  two  of  these  metals  placed 
in  an  acid  liquid  will  produce  a  current  of  electricity.  From 
the  law  just  given,  a  zinc-copper  couple  will  be  much  strong- 
er than  a  zinc-iron  one.  The  cause,  too,  of  the  efficiency 
of  the  zinc-carbon  (Bunsen's)  and  zinc-platinum  (Grove's) 
batteries  is  evident.  The  order  of  the  metals  depends  upon 
the  nature  of  the  exciting  liquid. 

Batteries  are  arranged  differently  for  different  purposes ;  the  decompos- 
ing power  or  intensity  of  galvanism  depends  upon  the  number  of  the  plates, 
while  its  quantity  and  power  of  generating  heat  and  magnetism  (Chapter 
XX)  depend  upon  their  extent  of  surface.  When  great  heat  is  wanted, 
all  the  zinc  plates  are  connected  together,  so  as  in  effect  to  form  one  great 
plate,  and  the  same  is  done  with  the  copper  plates.  This  is  quite  different 
from  the  arrangement  represented  in  Fig.  328,  which  is  calculated  for  the 
production  of  chemical  decomposition  and  for  giving  shocks. 

265.  Characteristics  of  Voltaic  Electricity. — Several  pecu- 


GALVANISM.  387 

liarities  distinguish  voltaic  electricity  from  that  developed 
by  friction.  Frictional  electricity  is  much  more  intense, 
and  that  produced  by  chemical  action  is  greater  in  quan- 
tity; the  latter  flows  in  a  current  as  fast  as  produced, 
while  the  former  is  discharged  with  a  kind  of  explosion. 
The  difference  has  been  compared  to  that  between  a  fire 
running  through  a  long  train  of  gunpowder  and  the  firing 
of  a  mass  of  it  at  once.  Another  peculiarity  of  voltaic  elec- 
tricity is  that,  unlike  frictional  electricity,  it  will  not  pass 
through  non-conductors.  Thus  you  can  handle  the  wires 
used  for  communication  without  receiving  a  shock;  but 
this  is  impossible  with  frictional  electricity.  This  property 
makes  voltaic  electricity  eminently  suitable  for  telegraph- 
ing. As  it  flows  with  a  steady  current,  its  action  can  be  con- 
trolled at  the  receiving  station;  and  it  pertinaciously  ad- 
heres to  the  wires  until  it  reaches  its  destination.  The  rain 
may  patter  upon  the  wire  without  sensibly  diminishing  the 
electricity ;  birds  may  rest  upon  it  and  receive  no  shock. 
The  principal  effects  of  galvanism  are  the  production  of 
heat  and  light,  as  well  as  of  magnetism  (Chapter  XX),  and 
the  decomposition  of  chemical  compounds.  Besides  these, 
it  has  a  peculiar  effect  on  the  nervous  system  of  animals. 
These  effects  we  will  now  describe  in  the  order  named. 

266.  Heating  Effects  of  Galvanism. — Galvanism  is  capable 
of  producing  the  most  intense  heat  known.  By  means  of 
a  battery  comprising  a  number  of  large  cups,  many  of  the 
most  refractory  substances  known  may  be  fused  and  even 
volatilized.  Galvanic  electricity,  unlike  frictional,  does  not 
run  over  the  surface  of  conductors ;  and,  consequently,  if 
the  terminal  wires  of  a  large  battery  be  connected  by  a 
wire  of  very  small  diameter,  the  resistance  offered  by  the 
latter  to  the  passage  of  the  electrical  current  causes  great 
heat ;  the  wire  becomes  red-hot,  and  may  even  melt.  If 
two  leaves  of  gold  are  suspended  at  the  ends  of  two  rods 


388 


NATURAL   PHILOSOPHY. 


in  a  jar,  Fig.  329,  and  the  rods  connect* 
ed  with  the  terminal  wires,  ov poles,  of 
a  large  battery,  they  will  move  towards 
each  other  until  they  touch,  and  then 
melt,  or  possibly  burn  up  with  a  brill- 
iant flash.  This  experiment,  contrived 
by  Sir  Humphry  Davy,  also  shows  that 
galvanism  as  well  as  frictional  electric- 
ity causes  attraction  (§  24]). 

That  this  attraction  is  really  the  ef- 
fect of  galvanism  may  be  shown  by 
suddenly  cutting  oif  the  connection 
between  the  gold  leaves,  and  the  battery  before  they  come 
in  contact;  they  will  at  once  fall  back  to  their  original 
position. 

Leaves  of  various  metals  may  be  burned  by  the  same  force,  each  metal 
giving  a  different  color  to  the  flame.  If  you  place  a  piece  of  silver  on  a 
lump  of  charcoal  connected  with  one  pole  of  a  battery,  and  bring  the  other 
pole  armed  with  a  charcoal  pencil  in  contact  with  it,  the  silver  will  burn 
most  brilliantly,  as  seen  in  Fig.  330.  The  burning  of  iron  by  galvanic  elec- 
tricity is  very  splendid.  Take  a 
glass  partly  filled  with  mercury, 
and,  immersing  one  pole  of  a  bat- 
tery, bring  a  thin  piece  of  steel  or 
iron  connected  with  the  other  pole 
in  contact  with  the  mercury,  and 
the  iron  will  take  fire,  throwing  out 
bright  sparks,  the  mercury  being 
rapidly  volatilized  by  the  great 
heat  produced.  Almost  every  substance  can  be  fused,  burned,  or  volatilized 
between  the  poles  of  a  powerful  battery.  Even  platinum,  which  can  with- 
stand the  heat  of  the  hottest  furnace,  can  be  fused.  The  heat  thus  pro- 
duced has  been  applied  in  firing  gunpowder  in  blasts,  and  this  is  sometimes 
done  even  under  water.  The  powder,  enclosed  in  a  vessel,  has  a  fine  plati- 
num wire  running  through  it,  which  is  connected  at  its  two  ends  with  two 
wires  that  extend  to  the  galvanic  apparatus.  The  battery  being  put  in 
operation,  the  moment  the  connection  is  established  the  platinum  becomes 


Fig.  330. 


GALVANISM. 


389 


red-hot  and  fires  the  powder.    The  battery  may  be  at  a  great  distance  from 
the  powder,  and  yet  the  requisite  effect  is  produced. 

267.  Production  of  Light. — Whenever  the  heat  produced 
by  galvanism  becomes  very  intense,  it  is  accompanied  by 
light,  as  shown  in  the  preceding  section. 

Generally  speaking,  the  production  of  light  is  the  result 
of  chemical  action,  as  will  be  shown  in  Part  II.,  Chem- 
istry. But  galvanism  may  produce  light  without  any 
chemical  change  taking  place  at  the  point  where  the  light 
is  seen.  For  example,  if  points  of  charcoal  be  fastened  to 
the  ends  of  the  two  wires  of  a  powerful  galvanic  battery, 
a  splendid  light  will  appear  when  the  charcoal  points  are 
brought  in  contact.  The  charcoal  does  not  burn  up,  but 
is  merely  mechanically  transferred  from  one  pole  to  the 
other  in  an  incandescent  state.  That  this  light  is  not  the 
result  of  a  combustion  of  the  charcoal  may  be  proved  in 
two  ways.  First,  the  moment  that  the  connection  is  bro- 
ken the  light  goes  out,  and  there  is  no  glowing  of  the 
charcoal,  as  there  would  be  if  it  had  been  set  on  fire. 
Then,  again,  the  light  can  be  produced  in  a  vacuum,  as 
represented  in  Fig.  331.  Here  the 
light  is  in  a  vessel  which  has  been 
exhausted  by  the  air  -  pump,  and 
there  can  be,  of  course,  no  combus- 
tion without  air.  The  same  splen- 
did light  can  be  obtained  with  the 
charcoal  points  under  water.  Al- 
though there  is  no  combustion  of 
the  charcoal,  nevertheless  the  elec- 
tric light  is  indirectly  the  result  of 
combustion.  As  will  be  explained 
in  Part  II.,  Chemistry,  the  oxidation 
of  the  zinc  in  the  battery  is  really  a 
combustion.  These  facts  illustrate 

R 


Fig.  331. 


300  NATURAL   PHILOSOPHY. 

again  the  close  connection  between  the  forces  of  heat,  light, 
electricity,  and  chemical  attraction,  to  which  we  have  al- 
ready referred  (§§  165  and  212).  The  chemical  action  in 
the  battery  cup  generates  electricity,  which  itself  produces 
both  heat  and  light.  The  transmission  of  force  is  very 
wonderful. 

268.  Chemical  Effects. — Galvanic  electricity  not  only  orig- 
inates in  chemical  action,  but  it  also  produces  chemical  ac- 
tion between  its  poles.  A  great  variety  of  compound  sub- 
stances can  be  decomposed,  and  simple  substances  can  be 
made  to  unite  and  form  compounds.  In  the  production  of 
the  electrical  light  there  is  no  chemical  effect,  for  there  is 
neither  decomposition  nor  composition.  In  fusing  metals  or 
volatilizing  them,  there  is  no  chemical  action,  for  they  are 
not  made  to  enter  into  combination  with  anything,  but  re- 
main simple  elements,  having  their  form  alone  changed. 
But  when  any  metal  is  actually  burned,  there  is  chemical  ac- 
tion, for  the  metal  unites  with  the  oxygen  of  the  air.  In  the 
fusing  of  platinum  there  is  no  chemical  action,  for  it  is  not 
oxidized.  Mercury  is  so  volatile  that  it  merely  flies  off  in 
vapor,  and  is  condensed  again  in  liquid  form  in  the  cool  air. 

Most  interesting  results  follow  when  water  is  submitted 
to  the  action  of  voltaic  electricity.  This  liquid,  usually  re- 
garded as  a  simple  substance,  is  actually  a  compound  of 
uvo  gases,  oxygen  and  hydrogen,  in  definite  proportions, 
botli  as  regards  weight  and  volume.  It  is  readily  decom- 
posed into  these  gases  by  a  battery  of  two  cups  constructed 
on  Grove's  or  Bunsen's  plan.  Fig.  332  shows  a  form  of  ap- 
paratus which  permits  the  gases  to  be  collected  separately. 
Through  the  bottom  of  a  glass  vessel  are  introduced,  wa- 
ter-tight, two  platinum  wires,  a  b  and  a  c.  Over  each  of 
these  wires  a  tube,  with  its  upper  end  closed,  is  placed. 
The  tubes  and  the  dish  are  filled  with  water,  which  is 
sli^htlv  acidulated  in  order  to  make  it  a  better  conductor. 


GALVANISM. 


391 


Fig.  332. 

If  the  wire  a  c  be  connected  with  the  positive  pole  of  a 
battery,  and  the  wire  a  b  with  its  negative  pole,  some  of 
the  water  will  be  decomposed,  and  the  resulting  gases,  oxy- 
gen and  hydrogen,  will  collect  in  the  tubes,  e  and  /  respec- 
tively, driving  the  water  down  in  them.  Twice  as  much  gas 
accumulates  in  /"as  in  e,  water  being  composed  of  two  vol- 
umes of  hydrogen  and  one  of  oxygen.  As  the  decomposi- 
tion of  the  water  proceeds,  the  hydrogen,  being  electro- 
positive, goes  to  the  negative  pole,  while  the  oxygen,  being 
electro-negative,  goes  to  the  positive  pole.  This  is  in  ac- 
cordance with  the  law  that  substances  charged  with  op- 
posite electricities  attract  each  other  (§  243).  After  a  suf- 
ficient amount  of  the  gases  has  collected,  cautiously  re- 
move the  tube  e,  closing  its  mouth  with  your  finger,  and, 
turning  it  upside  down,  introduce  into  it  a  slip  of  wood 
with  a  spark  on  its  end — the  wood  will  burst  into  a  flame, 
showing  that  the  gas  is  oxygen.  If  you  then  remove  /, 
and  apply  a  light  to  its  mouth,  the  gas  will  ignite  and 
burn  with  a  pale  flame.  Or  if  you  mingle  with  it  an  equal 
quantity  of  atmospheric  air,  and  then  apply  the  light,  an 


392  NATURAL   PHILOSOPHY. 

explosion  will  ensue — these  phenomena  being  characteristic 
of  hydrogen.  You  will  become  familiar  with  the  proper- 
ties of  these  gases  in  Part  II. ,  Chemistry. 

Explanation.  —  Let  us  now  examine  the  reason  why  the  gases  in  the 
above  experiments  accumulate  separately.  If  decomposition  of  the  wa- 
ter took  place  at  both  poles,  either  both  gases  would  collect  in  each  tube, 
or  portions  of  the  gases  must  pass  each  other  in  taking  their  places.  But 
neither  of  these  results  occurs.  Not  a  particle  of  one  gas  is  found  mixed 
with  the  other — one  tube  contains  pure  hydrogen  and  the  other  pure  oxy- 
gen. And  there  is  no  appearance  of  any  bubbles  of  gas  passing  between 
the  poles.  Though  the  gases  collect  briskly,  the  water  is  perfectly  quiet, 
which  would  not  be  the  case  if  gases  were  passing  through  it.  These 
things  being  so,  we  must  conclude  that  all  the  oxygen  is  produced  at  the 
positive  pole,  and  all  the  hydrogen  at  the  negative.  Let  us  see  how  this 
can  be.  Suppose  the  decomposition  begins  in  a  particle  of  water  at  the 
positive  pole,  an  atom  of  oxygen  uniting  with  the  zinc.  This  will,  of 
course,  leave  two  atoms  of  hydrogen  free.  Now,  what  becomes  of  the 
atoms  of  hydrogen  ?  Do  they  travel  over  to  the  other  pole?  No.  It  is 
supposed  that  they  seize  an  atom  of  oxygen  from  the  adjoining  molecule  of 
water,  forming  with  it  a  particle  of  water.  This  sets  two  more  atoms  of 
hydrogen  free,  which  take  an  atom  of  oxygen  from  the  next  molecule  of 
water,  and  so  on  through  a  chain  of  particles  extending  to  the  negative 
pole.  You  see  what  the  result  will  be  at  this  pole.  The  last  atoms  of  hy- 
drogen set  free  have  no  particle  of  water  to  take  oxygen  from,  and  hence 
the  gas  collects  in  the  tube. 

269.  Electrolysis.  —  A  great  many  other  substances  be- 
sides water  can  be  decomposed  by  galvanism,  each  of  the 
constituents  appearing  at  the  pole  opposite  to  it  in  elec- 
trical character.  Thus  oxygen,  chlorine,  iodine,  sulphur, 
and  the  various  acids,  being  electro-negative,  are  attracted 
by  the  positive  pole;  while  hydrogen,  potassium,  sodium, 
copper,  silver,  lead,  and  the  various  bases  are  attracted  by 
the  negative  pole.  It  was  by  galvanism  that  Sir  Humphry 
Davy  made  his  grand  discovery  that  the  alkalies  and  earths 
are  oxides  of  metals.  Great  must  have  been  the  joy  of  the 
discoverer  when,  as  he  subjected  the  potash  to  the  action 


GALVANISM.  393 

of  the  battery,  he  saw  the  metallic  globules  of  potassium 
appear,  turning  the  conjecture  which  had  so  long  burdened 
his  mind  into  reality  in  an  instant.  The  process  of  decom- 
posing chemical  substances  by  voltaic  electricity  has  re- 
ceived the  name  electrolysis. 

270.  Electrotyping.  —  Solutions  of  chemical  compounds 
are  also  decomposed  by  galvanism ;  sulphate  of  copper,  for 
example,  submitted  to  the  action  of  an  electric  current  is 
separated  into  its  constituents,  and  metallic  copper  is  de- 
posited at  one  pole  (negative)  of  the  battery.  This  has 
given  rise  to  an  important  branch  of  applied  science  called 
electro-metallurgy,  whereby  one  metal  may  be  coated  with 
a  uniform  and  brilliant  film  of  another  (plating)  or  a  fac- 
simile of  a  metallic  mould  may  be  obtained.  The  latter 
process  is  termed  electrotyping.  The  arrangement  of  the 
battery  and  bath  is  represented  in  Fig.  333.  The  glass 
trough  contains  a  solution  of  sul- 
phate of  copper,  and  the  galvanic 
battery  is  of  the  form  known  as 
Smee's,  in  which  platinized  silver  is 
used  in  place  of  copper.  Suppose 
we  wish  to  obtain  a  copy  in  copper 
of  one  of  the  faces  of  a  medal.  Cov- 
ering with  wax  all  the  medal  except 
that  part  which  is  to  be  copied,  we 

immerse  it  in  the  liquid  attached  to  the  negative  wire,  Z, 
and  attach  a  piece  of  copper,  C,  to  the  positive  wire,  S.  Ob- 
serve now  what  takes  place.  The  sulphate  of  copper  is 
decomposed  by  the  voltaic  current,  the  copper  going  to  the 
negative  pole  (§  269),  and  therefore  being  deposited  on  the 
medal,  N.  The  sulphuric  acid,  on  the  other  hand,  goes  to 
the  positive  pole,  and,  finding  copper  there,  unites  with  it, 
forming  sulphate  of  copper.  This  explains  the  use  of  the 
lump  of  copper.  It  is  for  the  purpose  of  keeping  a  good 


394  NATURAL   PHILOSOPHY. 

supply  of  sulphate  of  copper  in  the  liquid.  We  obtain  in 
this  way,  however,  only  a  mould  of  the  medal ;  but  by  pur- 
suing the  same  course  with  the  mould  we  get  the  fac-sirnile. 
So  in  obtaining  a  fac-simile  of  a  page  of  printer's  type  or 
of  an  engraving  a  mould  is  first  made.  This  is  commonly 
done  with  plaster  of  Paris  or  wax,  and  then  the  mould  is 
electrotyped.  We  get  in  this  way  a  thin  coat  of  copper 
showing  distinctly  every  line  of  the  types  or  the  engraving. 
Then  to  fit  this  for  use  melted  lead  is  poured  into  the  back 
of  it,  so  as  to  make  the  thin  coat  a  firm  plate.  Since  the 
wax  and  the  plaster  are  not  good  conductors,  we  are 
obliged  to  coat  the  surface  of  the  mould  with  finely  pow- 
dered graphite  in  order  to  secure  the  deposit  of  the  copper. 

271.  Gilding  and  Silver-Plating.  —  Common  metals  may 
be  plated  with  silver,  or  silver  and  other  metals  may  be 
gilded,  by  this  process  of  electrotyping.  Suppose,  for  ex- 
ample, that  a  silver  spoon  is  to  be  gilded.  It  is  attached  to 
the  negative  pole  of  a  battery,  and  immersed  in  a  solution 
of  chloride  of  gold,  while  a  plate  of  gold  is  attached  to  the 
positive  pole.  The  chloride  is  decomposed,  and  as  fast  as  the 
gold  is  deposited  upon  the  silver  the  chlorine  set  free  unites 
with  some  of  the  gold  plate,  thus  keeping  the  solution  of 
the  chloride  of  uniform  strength.  The  chlorine  is  the  car- 
rier of  the  gold  to  the  pole  connected  with  the  silver  in  the 
same  way  that  the  sulphuric  acid  is  the  carrier  of  the  cop- 
per in  the  process  of  copying  medals  described  in  §  270. 

The  arrangement  of  a  bath  for  silver-plating  a  number 
of  articles  at  one  operation  is  shown  in  Fig.  334.  The 
trough  C  contains  a  solution  of  cyanide  of  silver,  in  which 
are  suspended  the  silver  plates  o'  o  and  o'  o,  connected  with 
the  positive  pole  of  the  battery,  E  E,  and  the  pitchers  and 
other  articles  connected  with  the  negative  pole.  The  oper- 
ation proceeds  exactly  as  with  the  gold-plating  above  de- 
scribed. Electroplating,  as  illustrated  in  the  processes  men- 


395 


Fig.  334. 

tioned  and  in  others,  is  one  of  the  most  beautiful  and  valu- 
able presents  which  science  has  made  to  the  arts. 

272.  Physiological  Effects  of  Galvanism. — The  muscular 
contractions  produced  by  galvanism  have  been  already  al- 
luded to  in  speaking  of  Sulzer's  and  Galvani's  experiments. 
The  force  of  the  contraction  depends  upon  the  number  of 
the  pairs  of  plates,  and  not  upon  their  size.  The  shock 
received  from  a  galvanic  battery  is  not  like  the  sharp  ancl 
instantaneous  one  given  by  frictional  electricity,  but  it  is 
more  of  a  continued  sensation.  It  is  felt  only  at  the  mo- 
ment when  contact  is  made  or  broken,  a  steady,  continuous 
current  being  maintained  so  long  as  the  connection  exists.. 
With  a  battery  of  some  hundred  couples,  as  the  pairs  are 
termed,  the  shock  is  painfully  severe,  and  may  be  even  fa-1 
tal.  As  muscular  contractions  were  produced  in  Galvani's 
dead  frogs,  so  can  they  be  produced  in-  the  human  subject 
after  death.  Because,  although  the  body  may  be  dead  as  a 
whole,  as  a  system  of  organs,  some  of  the  properties  of  life 
still  remain  in  some  of  its  parts.  The  irritability  of  the 
muscles  is  such  a  property,  and  it  is  through  this  that  the 
culprit  who  has  been  hanged  can  be  galvanized  into  appar- 
ent life — the  countenance  exhibiting  frightful  contortions, 
and  the  limbs  being  thrown  violently  about.  Galvanism 


396  NATURAL  PHILOSOPHY. 

has  also  been  used  successfully  in  cases  where  animation 
was  suspended,  but  not  destroyed ;  the  muscles  of  respira- 
tion and  the  heart,  which  had  ceased  to  act,  being  awakened 
again  into  action  by  the  stimulus  of  the  electric  current. 
Physicians  sometimes  employ  galvanism  in  certain  diseases 
with  beneficial  effects. 


QUESTIONS. 

257.  What  is  said  of  the  history  of  galvanism  ?  What  was  Sulzer's  ob- 
servation ?  Describe  Galvaui's  experiment.  Wfiat  was  his  explanation  ? 
— 258.  Describe  the  construction  of  Volta's  pile.  How  does  it  operate  ? 
What  was  Volta's  explanation  ?  What  is  said  of  the  different  names  given 
to  galvanism  ? — 259.  Describe  the  action  of  the  voltaic  circle.  Explain  the 
chemistry  of  this  action. — 260.  What  is  the  advantage  of  amalgamatingthe 
zinc  ?  What  is  said  of  Davy's  plan  of  protecting  the  sheathing  of  vessels  ? 
— 2G1 .  What  is  said  of  polarity  in  galvanism  ?  What  is  meant  by  the  terms 
electro-negative  and  electro-positive  ?— 262.  In  what  other  ways  may  gal- 
vanism be  produced?  —  263.  Describe  Volta's  "Crown  of  Cups."  How 
does  it  operate  in  generating  galvanism  ?  Describe  the  modification  known 
as  the  trough  battery.  —  264.  What  is  said  of  other  forms  of  batteries  ? 
Describe  Daniell's.  Also  Grove's  and  Bunsen's.  What  determines  the 
choice  of  metals  used  in  a  battery  ?  Upon  what  does  the  intensity  and 
the  quantity  of  galvanism  depend  ? — 265.  What  are  the  chief  characteris- 
tics of  galvanism  ?  What  are  its  principal  effects? — 266.  Give  examples  of 
the  heating  effects  of  galvanism.  Describe  Sir  Humphry  Davy's  experi- 
ment. Show  how  silver  may  be  burned.  How  may  steel  be  burned  by  the 
heat  of  galvanism  ?  What  is  said  of  firing  gunpowder  by  electricity  ? — 
267.  What  is  said  of  the  production  of  light  ?  What  proof  is  given  that 
the  light  does  not  result  from  combustion  ?  Where,  however,  does  the 
combustion  really  take  place  ? — 2£8.  Mention  some  of  the  chemical  effects 
of  voltaic  electricity.  Describe  the  apparatus  used  for  the  decomposition 
of  water.  How  does  the  decomposition  proceed?  How  are  the  gases  ex- 
amined ?  Explain  why  the  gases  accumulate  at  separate  points.  What 
substances  are  attracted  to  the  positive  pole  ?  What  to  the  negative  pole  ? 
— 269.  What  remarkable  discoveries  were  made  by  Sir  Humphry  Davy  by 
means  of  voltaic  electricity?  What  is  meant  by  electrolysis?— 270.  What 
is  meant  by  electro-metallurgy?  Explain  the  manner  of  producing  an  elec- 
trotype.— 271.  How  is  electro-plating  conducted?  Describe  the  method 


MAGNETISM.  397 

of  silver-plating. — 272.  What  is  said  of  the  physiological  effects  of  galvan- 
ism? How  is  the  human  body  affected  by  galvanism?  How  are  dead 
bodies  affected  ? 


CHAPTER  XX. 

MAGNETISM. 

273.  Natural  Magnets. — Many  centuries  ago,  a  certain  ore 
of  iron  was  discovered  possessing  the  property  of  attract- 
ing pieces  of  common  iron  or  steel.  This  iron  ore,  some- 
times called  loadstone,  has  also  the  power  of  communicat- 
ing its  peculiar  properties  to  other  pieces  of  iron.  These 
are  called  magnets ;  and  the  force  residing  in  them,  mag- 
netism. These  facts  were  probably  considered  at  first  as 
mere  curiosities,  and  the  world  was  slow  to  find  out  their 
great  value.  It  has  been  recently  discovered  that  in  mag- 
netism we  have  one  of  the  great  forces  of  the  earth ;  and 
even  now  we  know  probably  but  little  of  the  real  extent 
and  variety  of  its  action.  New  and  important  discoveries 
are  frequently  made  in  regard  to  the  agency  and  the  laws 
of  this  mysterious  power,  and  its  connections  with  the  oth- 
er grand  forces  of  nature. 

The  terms  magnet  and  magnetism  come  from  the  fact 
that  the  loadstone  was  first  found  near  Magnesia,  an  an- 
cient village  in  Asia  Minor.  This  ore  5s  an  oxide  of  iron, 
differing  in  constitution  from  ordinary  iron  rust,  and  com- 
monly called  magnetite.  It  occurs  in  abundance  in  the 
iron-mines  of  Sweden,  England,  and  America.  Large  beds 
of  it  are  worked  in  New  York,  New  Jersey,  and  elsewhere 
in  the  United  States  and  Canada. 

The  property  of  attracting  iron,  possessed  by  magnets, 
may  be  exhibited  in  many  different  ways.  A  magnet 
brought  near  a  heap  of  iron  filings,  needles,  tacks,  etc., 


398  NATUKAL  PHILOSOPHY. 

will  attract  them  and  cause  them  to  adhere  to  it  when  re- 
moved. Many  toys  for  children  depend  upon  magnetism 
for  their  attractiveness.  Thus  the  toy  fishes  and  swans 
follow  the  small  iron-bar  magnet  because  a  small  piece  of 
iron  is  concealed  in  their  heads.  Simple  experiments  with 
a  magnet  soon  convince  us  that  the  nearer  the  magnet  and 
the  iron  are  to  each  other,  the  stronger  the  attraction.  In- 
deed, the  attractive  influence  is  governed  by  the  same  law 
in  regard  to  distance  as  the  common  attraction  of  matter — 
viz.,  it  is  inversely  as  the  square  of  the  distance.  The  at- 
traction also  is  mutual — the  iron  attracting  the  magnet  as 
much  as  the  magnet  does  the  iron. 

274.  Polarity  of  the  Magnet.  —  Every  magnet  has  two 
poles,  one  at  each  extremity,  about  which  the  chief  power 
resides.  For  this  reason,  if  a  magnet  be  rolled  in  iron  fil- 
ings, these  collect  about  the  ends,  as  represented  in  Fig. 
335.  There  is  a  diminution  of  attraction  from  the  ends  to 


the  middle  line,  which  is  called  the  neutral  line.  These 
poles  are  called  north  and  south  poles,  because  if  a  magnet 
be  suspended,  or  be  supported  upon  a  pivot,  so  that  it  can 
revolve,  it  will  take  a  north  and  south  direction,  one  of  its 
ends  invariably  pointing  towards  the  -north.  Fig.  336  rep- 
resents a  magnet  supported  upon  a  pivot.  If  a  magnet  be 
broken,  each  piece  becomes  immediately  a  perfect  magnet, 
having  poles  of  its  own. 

275.  Magnetism  by  Induction. — The  magnet  in  exerting 
its  attraction  temporarily  makes  a  magnet  of  the  body 
attracted.  Actual  contact  is  not  necessary  to  produce 


MAGNETISM. 


399 


.336. 


this  result.  Thus  if  a  large 
iron  key  be  brought  only 
very  near  to  a  powerful  mag- 
net, it  will  support  small  keys, 
as  represented  in  Fig.  337. 
When  the  key  is  moved  away 
from  the  magnet,  the  keys 
attached  to  it  fall.  You  see 
the  analogy  to  the  induction 
of  electricity  noticed  in  §  247. 
The  two  ends  of  the  body  in 
which  the  influence  is  induced 
are  in  opposite  states.  If  the 
end  of  the  magnet  to  which 
the  first  key  is  near  or  at- 
tached be  the  north  pole,  the  end  of  the  key  next 
to  the  magnet  will  be  the  south  pole,  and  its  farther 
end  the  north  pole.  The  same  is  the  case  with  the 
small  key  attached  to  the  end  of  the  large  one.  And 
if  a  nail  should  hang  from  the  small  key,  and  a  nee- 
dle from  the  nail,  both  of  these  would  have  the  same 
polarities.  But  all  this  would  be  reversed  if  the 
1S'  '  large  key  were  attached  to  the  south  pole  of  the 
magnet.  In  this  case  the  upper  end  of  each  of  these  articles 
would  be  the  north  pole,  and  its  lower  end  the  south  pole. 

As  a  result  of  these  experiments  we  learn  that  like  poles 
repel  and  unlike  attract.  But  this  law  can  be  more  striking- 
ly illustrated.  If  a  magnet  be  placed  on  a  pivot,  as. in  Fig. 
336,  and  another  magnet  be  brought  near  it,  attraction  or 
repulsion  will  be  manifested  according  to  the  mode  of  pre- 
sentation. If  a  north  pole  be  presented  to  a  north  pole,  or 
a  south  to  a  south,  repulsion  will  be  the  result.  But  if  a 
north  pole  be  presented  to  a  south,  or  a  south  to  a  north, 
then  attraction  will  be  manifested. 


400 


NATUKAL  PHILOSOPHY. 


Magnetic  Curves. — The  polarity  of  magnetism  causes  a 
very  singular  arrangement  of  iron  filings  when  gently  agi- 
tated upon  a  sheet  of  paper  over  a  magnet,  as  represented 
in  Fig.  337.     The  production  of  these  curves  is  owing  en- 
^__^^m^^^__^TT^__^_^^^  tirely  to  the  fact  that 


each  bit  of  filing  is 
5!;  I  polarized  by  the  bit 

next  preceding  it  in 
W  the  row,  reckoning 
jpsj  from  the  magnet 
if  outward,  the  nearest 
I  one  in  each  row  de- 


riving its  magnetic 
state  from  the  mag- 
net itself.  Since  the  chief  power  resides  in  the  ends  of 
the  magnet,  it  is  easy  to  see  how  such  a  disposition  of  the 
lines  of  magnetic  filings  is  effected.  These  curves  may  be 
beautifully  and  curiously  varied  by  arranging  several  mag- 
nets under  the  paper. 

276.  Artificial  Magnets. — As  already  mentioned,  the  pow- 
er residing  in  the  loadstone  can  be  communicated  readily 
to  iron  and  steel.  Though  soft  iron  becomes  magnetic  more 
readily  than  steel,  it  does  not  retain  the  power  so  well,  and 
the  latter  is  therefore  used  in  making  artificial  magnets. 
When  a  magnet  imparts  its  magnetic  influence,  it  loses  none 
of  its  own  power,  whether  it  be  an  original  loadstone  or  an 
artificial  magnet.  There  are  many  ways  of  imparting  mag- 
netism permanently  to  steel,  of  which  we  will  notice  only 
two.  If  you  wish  to  magnetize  a  bar  or  needle,  pass  one 
pole  of  a  magnet  from  one  end  of  it  to  the  other  a  number 
of  times,  always  in  the  same  direction.  A  more  effectual 
way  is  to  take  two  magnets,  and,  placing  the  south  pole  of 
one  and  the  north  pole  of  the  other  in  contact  over  the 
middle  of  the  bar  or  needle,  draw  them  slowly  and  steadily 


MAGNETISM. 


401 


apart  towards  the  opposite  ends.  This  process  must  be  re- 
peated several  times. 

Horseshoe  Magnets. — One  of  the  most  common  forms  of 
the  magnet  is  the  horseshoe  magnet,  Fig.  338.  A 
piece  of  soft  iron,  called  the  keeper,  is  attached  to 
the  end  of  this,  held  there  by  attraction.  So  long 
as  it  is  suffered  to  remain  there  it  is  itself  a  magnet, 
having  its  north  pole  (-f )  attached  to  the  south 
pole  (— )  of  the  magnet  which  holds  it,  while  the 
reverse  is  the  case  with  its  south  pole.  The  ob-  ^ 
ject  of  the  keeper  is  to  preserve  the  power  of  the  Flg>33bt 
instrument.  Indeed,  it  is  found  that  the  exertion  of  the 
magnet's  power  not  only  preserves  but 
actually  increases  it.  If,  therefore,  you 
attach'  to  a  magnet  a  keeper  having  a 
hook,  as  shown  in  Fig.  339,  you  can  add 
to  the  weight  gradually  from  day  to  day, 
and  thus  considerably  augment  the  power 
of  the  magnet. 

277.  Magnetic  Needle.  —  The  magnetic 
needle  is  a  very  small  magnet  fixed  upon 
a  pivot.     On  account  of  its  property  of 
invariably  pointing  north  and  south,  it  is 
Fig.  339.  Of  great   use   to   sailors.     The   mariner's 

compass  is  a  round  box  having  a  magnetic  needle  balanced 
in  it,  and  provided  with  a  disk  of  cardboard  on  which  is 
drawn  a  circle  divided  into  thirty-two  parts,  as  shown  in 
Fig.  340.  These  divisions,  called  the  "  points  of  the  com- 
pass," have  received  names  with  reference  to  the  four  di- 
rections— north,  east,  south,  and  west.  The  original  com- 
pass was  a  rude  affair,  consisting  of  a  small  piece  of  load- 
stone laid  upon  a  cork  floating  in  water.  The  date  and 
place  of  its  first  use  are  unknown,  but  it  is  commonly  as- 
cribed to  the  Chinese. 


402 


NATURAL   PHILOSOPHY. 


Fig.  340. 

Declination  of  the  Needle. — The  declination  of  the  needle  is  its  deviation 
from  a  north  and  south  line.  There  are  comparatively  few  parts  of  the 
earth's  surface  in  which  there  is  no  deviation  from  this  line  to  the  east  or 
the  west.  "True  as  needle  to  the  pole"  has  become  a  proverb ;  and  when 
first  uttered,  it  was  supposed  to  be  founded  in  strict  truth  ;  modern  investi- 
gation, however,  has  shown  that  the  needle  not  only  varies  in  its  direc- 
tion in  different  localities,  but  that  even  its  variations  are  irregular.  This 
irregularity  in  the  declination  of  the  needle  was  observed  by  Columbus  in 
his  first  voyage  of  discovery,  and  it  occasioned  great  alarm  among  the  sail- 
ors, who,  as  Irving  states,  "thought  the  laws  of  nature  were  changing,  and 
that  the  compass  was  about  to  lose  its  mysterious  power."  Notwithstand- 
ing these  and  other  observations  of  a  similar  character,  little  attention  was 
paid  to  the  declination  of  the  needle  till  the  middle  of  the  seventeenth  cen- 
tury. But,  since  that  time,  extensive  records  of  its  declinations  at  dif- 
ferent localities  have  been  made,  and  tables  and  charts  have  been  con- 
structed exhibiting  them.  These  declinations  are  not  constant,  but  vary 
somewhat  every  day,  from  the  influence,  it  is  supposed,  of  the  sun  upon 
the  earth. 

Dip  of  the  Needle. — It  is  found  in  most  parts  of  the  earth  that  if  a  nee- 
dle be  balanced  before  it  is  magnetized,  and  then  suspended  in  the  same 
manner  after  magnetization,  one  end  will  dip  downward,  as  shown  in 


MAGNETISM.  403 

Fig.  341.  This  fact  was  discovered  by  Norman,  a 
London  optician,  in  1576.  lie  found  that  the  dip 
at  London  was  towards  the  north  at  an  angle  of  72°. 
In  pursuing  the  investigation  of  this  phenomenon 
it  was  found  that  going  from  the  north  towards 
the  equator  the  dip  constantly  lessened,  until  a 
point  was  reached  where  the  needle  was  horizontal. 
Then,  on  going  south  of  this,  a  reverse  dip  occurred 
towards  the  south  pole;  and  the  farther  south  the 
needle  was  carried,  the  greater  the  dip.  In  the 
north,  Captain  Ross,  in  1832,  came  to  a  locality 
north  of  Hudson's  Bay,  in  latitude  70°  5'  N.,  longi- 
tude 96°  45' W.,  where  the  magnetic  needle,  freely 
suspended,  was  in  a  vertical  line.  No  such  local- 
ity has  yet  been  discovered  towards  the  south 
pole. 

278.  The  Earth  a  Great  Magnet.  — 
You  can  readily  see,  from  all  that  has 
been  stated  in  regard  to  the  magnetic 
needle,  that  the  earth  acts  as  a  magnet.  The  dip  of  the 
needle  shows  that  the  two  poles  of  this  magnet  are  some- 
where near  the  north  and  south  poles  of  the  earth.  The 
locality  which  Captain  Ross  found  must  be  near  the  north 
pole  of  the  magnet  in  that  quarter  of  the  world.  The  ver- 
tical position  of  the  needle  is  analogous  to  the  straight 
lines  of  iron  filings,  shown  in  Fig.  337,  near  the  poles  of  the 
magnet.  And  it  is  easy,  also,  to  trace  the  analogy  between 
the  dip  of  the  needle  at  different  distances  from  what  is 
called  the  magnetic  equator  of  the  earth,  where  the  needle 
is  horizontal,  and  the  curves  extending  from  pole  to  pole. 
The  different  declinations  of  the  needle  and  the  differ- 
ent intensities  of  the  magnetic  force  in  different  localities 
corresponding  in  latitude  show  that  the  magnetic  force 
in  the  earth  is  irregular  in  distribution,  or  in  some  way 
its  influence  varies  much  in  different  parts  of  the  earth's 
crust. 


404  NATURAL  PHILOSOPHY. 

The  Earth  a  Magnetizer. — Since  the  earth  is  really  a  mag» 
net,  it  might  be  expected  to  impart  magnetism  by  induction 
as  other  magnets  do.  And  this  is  found  to  be  the  fact.  If 
you  hold  a  bar  of  soft  iron  in  the  direction  of  the  dip  of  the 
needle,  it  becomes  a  magnet — its  lower  end  being  the  north 
pole  and  its  upper  the  south.  That  this  is  so  can  be  ascer- 
tained by  bringing  a  small  magnetic  needle  near  each  end. 
No  eifect  of  this  kind  is  produced  when  the  bar  is  held  hori- 
zontally east  and  west.  Lightning-rods,  pokers,  upright 
iron  bars  in  fences,  etc.,  are  often  found  to  be  magnetized 
because  they  have  continued  so  long  nearly  in  the  required 
position  for  magnetization.  When  a  bar  of  iron  has  been 
magnetized  in  the  manner  indicated,  its  magnetism  may 
sometimes  be  rendered  permanent  by  giving  it  a  stroke 
with  a  hammer.  It  is  a  curious  and  inexplicable  fact  that 
this  vibration  of  the  particles  of  iron  should  have  this  eifect. 
But  though  such  vibration  helps  to  impart  magnetism,  it  is 
not  at  all  favorable  to  its  retention,  for  magnets  are  always 
injured  by  blows  or  falls,  or,  indeed,  any  rude  treatment. 
For  this  reason  care  is  requisite  in  removing  a  keeper  from 
a  magnet.  If  pulled  off  abruptly,  the  power  of  the  magnet 
is  lessened.  If  a  magnet  be  heated,  its  power  is  impaired ; 
but,  on  cooling,  it  returns.  A  red  heat,  however,  destroys 
it  completely. 

279.  Theory  of  Magnetism. — Magnetism  is  now  generally 
conceded  to  be  a  molecular  affection.  "For,  in  the  first 
place,  if  a  steel-bar  magnet  be  broken  in  two,  each  part  is 
as  complete  a  magnet  as  the  original ;  and,  however  often 
we  break  it,  the  minutest  fragment  is  a  perfect  magnet — 
showing  that  the  polarity  or  duality  of  character  which  the 
original  bar  possessed  is  equally  a  property  of  every  mole- 
cule." This  may  be  roughly  illustrated  by  reference  to 
Figs.  342,  343,  and  344,  in  which  the  north  and  south  poles 
of  each  molecule  in  a  magnet  are  represented  by  black  and 


MAGNETISM.  405 

white  parts  of  the  circular  particles 

composing  it.     In  Fig.  342,  some  of 

the  poles  are  turned  one  way  and 

some  another;  and  if  the  number  be  Pig. 342. 

equal,  their  effect  is  neutralized  and 
the  steel  has  no  magnetic  properties. 

In  the  next  figure  y°U  See  some  of  the 
molecules  turned  half-way  around, 

as  in  an  imperfectly  magnetized  bar; 
and,  finally,  in  Fig.  343,  we  have 
represented  a  complete  magnet,  in 
which  each  molecule  is  turned  in  NC)C)€)C€)C)CCS 
the  same  direction.  CCCCCCCC 

A  f         i.         T.  Fig.  344. 

A  second  reason  for  the  theory 

of  molecular  affection  is  that  "  when  a  bar  of  iron  is  mag- 
netized it  becomes  slightly  elongated,  while  its  width  is 
correspondingly  reduced,  just  as  if  there  were  a  re-arrange- 
ment of  the  molecules  among  themselves;  as  if  each  turned 
around  and  set  with  its  greatest  length  in  the  axis  of  the 
bar."  In  the  third  place,  as  mentioned  in  the  previous 
section,  heating  and  striking  a  magnet  destroy  its  power 
^— in  other  words,  disturb  its  molecular  arrangement. 

Diamagnetism. — It  was  formerly  thought  that  only  iron 
'ind  its  companion  metals  cobalt  and  nickel  were  affected 
by  magnetism;  but  the  researches  of  Faraday  have  shown 
that  all  bodies  are  affected  by  it  to  a  greater  or  less  degree. 
Experiments  with  a  very  powerful  magnet  proved  that  all 
bodies  may  be  divided  into  two  classes — the  paramagnetic 
which  are  attracted  by  a  magnet,  and  the  diamagnetic, 
which  are  repelled.  To  the  former  class  belong  iron,  nick- 
el, cobalt,  platinum,  oxygen  gas,  air,  etc.;  to  the  latter, 
bismuth,  mercury,  silver,  gold,  water,  alcohol,  wax,  sugar, 
wood,  leather,  bread,  and  a  great  variety  of  substances. 

280.  Electro-Magnetism. — In  1819,  Professor  Oersted,  of 


406  NATURAL  PHILOSOPHY. 

Copenhagen,  discovered  a  remarkable  relation  between  elec- 
tricity and  magnetism,  which  has  led  to  inventions  of  im- 
mense benefit  to  mankind.  His  first  observation  was  that* 
a  current  of  electricity  passing  over  a  wire  near  a  magnetic 
needle  affected  the  position  of  the  needle.  He  found  also 
that  iron  filings  would  adhere  to  a  wire  over  which  a  cur- 
rent of  electricity  is  passing,  just  as  they  do  to  a  magnet, 
dropping  off,  however,  as  soon  as  the  current  ceases  to  pass. 
Such  facts  led  to  a  great  variety  of  investigations  and  ar- 
rangements of  apparatus  by  Oersted  and  others.  Some  of 
these  we  will  indicate.  Let  a  wire  be  wound  in  a  spiral 

form,  leaving  the  two 
ends  of  the  wire  free, 
one  at  each  end  of  the 
coil.  Such  a  coil,  made 
with  a  great  length  of 
wire,  called  a  helix,  is 
represented  in  Fig. 
345.  Since  the  electric- 
ity is  to  pass  through 

Fig.  345.  all  the  length  of  the 

wire  uninterruptedly,  it  must  be  covered  with  some  non- 
conducting substance,  as  silk  or  cotton.  If  a  bar  of  iron  be 
introduced  into  the  helix,  and  the  wires  be  connected  with 
the  poles  of  a  battery,  the  iron  becomes  at  once  a  magnet, 
and  will  attract  various  iron  articles,  as  shown  in  the  fig- 
ure. But  the  moment  you  cut  off  the  connection  with  the 
battery,  these  articles  fall,  showing  that  the  magnetism 
of  the  iron  depends  wholly  upon  the  electric  current. 
A  magnet  thus  temporarily  made  is  called  an  electro* 
magnet,  and  the  power  thus  developed  is  called  electro- 
magnetism. 

Experiments  with  an  Electro-Magnet. — The  power  of  an 
electro-magnet  is  very  amusingly  exhibited  in  the  experi- 


MAGNETISM. 


407 


ment  represented  in  Fig.  346.  A 
bar  of  iron  having  the  common 
horseshoe  form  has  a  wire  coiled 
round  it.  If  the  connection  be 
made  with  the  two  poles  of  a 
battery,  on  bringing  it  near  to 
a  heap  of  nails  they  will  become 
attached  to  the  two  poles  of  the 
temporary  magnet  thus  formed, 
and  a  bridge  of  nails  will  be  con- 
structed between  them,  the  nails 
being  grouped  together  in  all 
kinds  of  positions,  as  the  magnetic  influence  extends  among 
them.  You  can,  however,  at  once  demolish  this  bridge  by 
cutting  off  the  connection  with  the  battery. 

A  Bar  Suspended  in  Air. — The  electro-magnetic  pow- 
er can  be  still  more  strikingly  exhibited.  Let  the  helix, 
A,  Fig.  347,  be  placed  vertically,  and  the  bar  of  iron,  B, 
be  held  upright  in  the  circular  space  in  the  helix.  On 
making  the  connection  with  the  battery  you  can  let  go  of 
the  bar,  and  it  will  remain  suspended,  being  held  there  by 
the  magnetic  power  created  by  the  electric  current.  If 
the  wire  in  the  coil  be  very  long,  and  the  battery  power- 
ful, a  considerable  weight  may  be  attached  to  the  bar  with- 
out making  it  fall.  "  Science,"  says  Professor  Porter,  in 
relation  to  this  experiment, 
"has  thus  realized  the  fable 
of  Mohammed's  coffin,  which 
Avas  said  to  have  been  mirac- 
ulously suspended  in  air." 

Fig.  348  represents  an  ap- 
paratus which  exhibits  elec- 
tro-magnetism very  prettily. 
Two  pieces  of  soft  iron,  when 
put  together,  form  a  ring,  d, 
and  each  piece  has  a  handle.  If  the  pieces  be 
put  together  with  the  coil,  c,  in  the  position  rep-  Fig.  348. 


B 


408 


NATUKAL  PHILOSOPHY. 


resented,  on  connecting  the  wires,  P  and  N,  with  a  battery  in  action,  mag- 
netism is  brought  into  play  and  the  adhesion  is  so  strong  as  to  resist  a  great 
force;  but  as  soon  as  the  connection  is  broken,  the  pieces  come  apart  at  once. 

The  attractive  power  of  electro-magnets  is  determined 
by  the  number  of  coils  of  insulated  wire  wound  around  the 
bar,  and  the  strength  of  the  current  of  electricity  passing 
through  these  wires.  Since  these  may  be  indefinitely  in- 
creased, the  magnetic  power  is  almost  unlimited.  Profess- 
or Henry  constructed  a  horseshoe  electro  -  magnet  which 
sustained  a  weight  of  over  2000  pounds,  and  other  experi- 
menters have  much  exceeded  this. 

.  281.  The  Electric  Telegraph.  —  In  the  electric  telegraph 
both  voltaic  electricity  and  electro-magnetism  are  brought 
into  our  service,  the  former  to  transmit  the  message  and 
the  latter  to  record  it.  To  effect  the  transmission  of  the 
galvanic  current  the  arrangement  is  essentially  the  same 
as  in  the  simple  voltaic  circle  (§  259).  Suppose  a  message 
is  to  be  sent  from  New  York  to  New  Haven.  Let  «,  Fig. 
349,  be  the  battery  at  New  York,  and  b  the  wire  communi- 
i 


eating  with  the  register,  c,  at  New  Haven.  Two  plates  of 
metal  are  buried  in  the  moist  earth  at  d  and  e,  each  having 
a  surface  of  several  square  feet.  These  are  connected  by 
wires  with  the  insulated  wire  between  the  two  places,  the 
,one  through  the  battery  at  New  York  and  the  other  through 
the  registering  electro-magnet  at  New  Haven.  The  cur- 


MAGNETISM. 


409 


rent  of  positive  electricity  from  the  battery  runs,  as  the 
arrows  indicate,  first  along  the  wire,  t>,  then  from  the  elec- 
tro-magnet at  c  down  to  the  plate  e,  then  through  the  earth 
to  the  plate  d,  and  up  to  the  place  of  beginning,  a.  It  was 
supposed  when  the  telegraph  was  first  projected  that  it 
would  be  necessary  to  have  two  wires  to  complete  the  cir- 
cuit; but  it  was  soon  found  that  the  earth  answered  per- 
fectly well  for  the  returning  current  with  the  arrangement 
of  plates  described.  The  control  which  the  telegrapher  has 
over  the  current  with  his  key,  as  described  below,  is  indi- 
cated at  i.  The  wire  is  there  represented  as  broken,  and  no 
effect  will  be  produced  at  c  until  the  operator  at  i  presses 
one  end  of  the  wire  down  upon  the  other  end. 

The  manner  of  recording  the  message  is  as  follows :  As 
before  stated,  voltaic  electricity  is  used.  This  is  generated 
at  the  place  from  which  the  message  is  sent,  and  passes 
over  the  wire  to  the  place  where  the  message  is  received. 
There  it  acts  upon  soft  iron  by  passing  through  coiled  wire, 
producing  the  modified  power  called  electro -magnetism. 
We  will  make  all  this  plain  to  vou  bv  describing  the  ma- 

•/  V 

chine  used  in  Morse'i  telegraph,  Fig.  350.     W  W  are  the 


410  NATURAL  PHILOSOPHY. 

wires  which  connect  with  the  station  from  which  the  mes- 
sage is  to  be  received,  and  these  connect  with  the  copper 
wire  coiled  round  the  horseshoe  of  soft  iron,  m  m.  Above 
the  magnet  is  a  lever,  a  I,  which  works  on  a  fulcrum  at  d. 
One  end  of  this  lever  has  a  steel  point,  s,  attached  to  it. 
JAt  c  is  an  arrangement  of  wheel- work,  the  object  of  which 
is  to  pass  along  regularly  a  narrow  band  of  paper, p,  in  the 
direction  of  the  arrows.  Observe  now  how  the  apparatus 
works.  When  the  electric  current  passes  through  the  coiled 
copper  wire  it  makes  a  magnet  of  the  iron,  m  m.  The  lever, 
a  /,  is  therefore  attracted  at  the  end,  a,  and  moves  down- 
ward. Of  course  the  end,  /,  moves  upward,  bringing  the  steel 
point,  s,  against  the  paper,  where  it  makes  a  mark.  The 
length  of  this  mark  depends  upon  the  length  of  time  the 
electricity  is  allowed  to  pass  along  the  coiled  wire,  for  the 
moment  that  it  is  shut  off  m  m  ceases  to  be  magnetic,  the 
"  keeper,"  a,  being  no  longer  attracted,  moves  upward,  and 
the  other  end,  /,  of  the  lever  moves  downward,  taking  the 
point,  s,  from  the  paper. 

In  order  to  make  the  marks  on  the  paper  of  different 
lengths,  there  is  a  contrivance  for  regulating  the  length  of 
time  that  the  current  shall  pass  through  the  coiled  wire. 
This  contrivance,  called  the  signal  key^  is  represented  in 

Fig.  351.  N  and  P  are  two  strips 
of  brass  connected  with  the  two 
wires  R  and  M,  of  which  M  comes 
from  the  battery.  The  end  of  the 
strip  N"  is  raised  a  little  above  the 
end  of  P.  So  long  as  they  do  not 
touch,  the  circuit  is  not  complete, 

and  no  electricity  passes.  But  if  the  operator  press  N 
down  upon  P,  the  circuit  is  established,  and  the  electricity 
passes  to  the  station  with  which  he  is  in  communication, 
and  there  acts  upon  the  apparatus  seen  in  Fig.  350.  Now, 


MAGNETISM. 


411 


the  longer  the  finger  presses  down  N  upon  P,  the  longer 
will  be  the  mark  on  the  paper  at  the  distant  station.  An 
operator  then  at  New  York,  for  example,  controls  by  this 
key  the  length  of  the  marks  made  on  the  paper  in  New 
Haven  or  any  other  place  with  which  he  is  communicating. 
You  can  readily  see  how  a  telegraphic  alphabet  can  be 
constructed  by  combinations  of  marks  of  different  lengths 
representing  different  letters  and  numerals.  Below  is  the 
alphabet  used  in  connection  with  Morse's  telegraph: 


A  -  — 

B 

C 

D  —  -  - 

E  - 

F 

G 

H 

I  -- 

J 

K 

L  

M 


N  — - 

O  -   - 

P 

Q 

R 

S 

T  — 

U 

V 

W 

X 

y 

Z 


Numerals. 

1 

2 

3 

4 

5 

6 - 

7 

8 

9 

0  


282.  Invention  of  the  Telegraph. — Great  as  is  the  credit  which 
should  be  awarded  to  our  countryman  Morse,  he  was  by  no  means  the  first 
inventor  of  a  telegraph.  There  was  one  invented  and  put  in  successful 
operation  more  than  a  century  ago  (in  1747)  in  London  by  Dr.  J.  Watson. 
It  was  over  two  miles  long,  and  the  earth  was  used  for  the  return  current. 
Frictional  electric'ity-  was,  of  course,  employed,  as  galvanism  .was  not  dis- 
covered till  about  half  a  century  after.  How  Dr.  Watson  contrived  to 
make  the  electricity  communicate  any  information  is  not  stated  by  Profess- 
or Silliman,  from  whose  work  on  Natural  Philosophy  the  facts  here  given 
are  taken.  The  year  following  the  construction  of  Watson's  telegraph 
Franklin  set  fire  to  spirits  of  wine  by  electricity  sent  across  the  River 
Schnylkill  on  a  wire,  using,  like  Dr.  Watson,  the  earth  to  complete  the  cir- 
cuit. He  made,  however,  no  attempt  to  apply  electricity  to  telegraphing. 
In  1774,  Le  Sage,  a  Frenchman,  constructed  a  telegraph  at  Geneva,  in 
which  he  used  twenty-four  wires  enclosed  in  glass  tubes,  which  were  buried 
in  the  earth,  each  wire  answering  to  some  letter  in  the  alphabet.  From 
this  time  there  were  various  trials  made  at  telegraphing,  but  only  partial 


412  NATURAL   PHILOSOPHY. 

success  attended  them,  until  the  year  1837,  when,  as  Professor  Silliman 
says,  "  almost  at  the  same  time  appeared  Morse  in  the  United  States,  Stein- 
heil  at  Munich,  and  Wheatstone  and  Cooke  in  England,  as  distinct  and  in- 
dependent claimants  for  the  honor  of  the  discovery."  But,  while  credit  is 
due  to  all  these,  Morse  stands  unmistakably  pre-eminent,  because  his  mode 
of  communication  and  record  at  the  receiving  station  is  entirely  original. 
For  nearly  one  hundred  years  there  were  scientific  men  earnestly  feeling 
after  the  result  which  Morse  had  the  privilege  of  fully  consummating,  arid 
how  much  influence  their  investigations  had  upon  his  mind  we  do  not  know. 
Neither  does  he  know  himself;  but,  with  the  truthfulness  and  modesty 
characteristic  of  the  honest  votary  of  science,  no  one  would  be  more  ready 
than  the  noble  inventor  to  acknowledge  this  influence.  Nay,  more  :  he  was 
directly  indebted  to  some  of  his  immediate  predecessors  for  discoveries 
which  were  essential  to  the  consummation  which  he  achieved.  For  ex- 
ample, Oersted's  discovery  of  electro-magnetism  supplied  Morse  with  the 
means  of  contriving  his  beautiful  mode  of  receiving  and  recording  messages. 
And  here  observe  the  distinction  between  discovery  and  invention.  Oersted 
was  a  discoverer,  Morse  an  inventor.  Oersted  discovered  a  great  fact  or 
principle,  and  Morse  found  or  invented  a  way  of  applying  this  principle  in 
telegraphing. 

Other  Applications  of  Electricity.  —  Another  volume 
equal  in  size  to  this  one  might  be  written  describing  the 
marvellous  phenomena  and  applications  of  electricity.  For 
an  account  of  the  Atlantic  telegraph  cable  and  its  method 
of  working,  of  the  various  machines  contrived  for  convert- 
ing electricity  into  motive  power  (known  as  electro-mag- 
netic engines),  and  of  many  other  recent  inventions,  such  as 
the  telephone,  we  must  refer  the  student  to  larger  treatises. 


QUESTIONS. 

273.  What  are  loadstones  ?  Where  do  they  abound  ?  What  is  said  of 
discoveries  in  magnetism  ?  Whence  come  the  terms  magnetism  and  mag- 
net ?  How  may  the  property  of  magnetism  be  exhibited  ?  What  is  snid 
of  the  attraction  of  magnetism  ?  What  laAv  is  there  in  regard  to  it?— 274. 
What  is  said  of  the  poles  of  amngnet? — 275.  What  of  magnetism  by  in- 
duction ?  What  is  said  of  attraction  and  repulsion  in  magnets  ?  Explain 


MAGNETISM.  413 

the  formation  of  the  curves  of  iron  filings  in  the  experiment  described. — 
27G.  How  may  artificial  magnets  be  made?  What  is  said  of  the  horse- 
shoe magnet  and  its  armature  ? — 277.  What  is  said  of  the  mngnetic  needle 
and  the  mariner's  compass  ?  What  is  the  declination  of  the  needle  ?  When 
was  it  first  observed  ?  What  is  said  of  observations  after  this  ?  What  is 
said  of  the  dip  of  the  needle  ? — 278.  What  is  said  of  the  earth  as  a  mag- 
net? What  of  it  as  a  magnetizer?  What  is  said  of  fixing  magnetism? 
What  of  impairing  it? — 279.  What  is  the  present  theory  of  magnetism? 
How  is  this  theory  supported?  What  is  meant  by  diamagnetism ?  In 
what  other  substances  besides  iron  does  magnetism  exist  ? — 280.  What  ob- 
servations did  Oersted  make  in  1819?  What  is  said  of  the  relation  between 
electricity  and  magnetism  ?  Describe  the  experiments  with  electro-mag- 
nets. What  determines  the  power  of  electro-magnets  ? — 281.  What  is  said 
of  the  forces  used  in  the  electric  telegraph  ?  Explain  the  principle  by 
which  the  galvanic  current  is  transmitted.  Explain  the  manner  of  record- 
ing the  message.  What  is  the  use  of  the  signal  key  ?  How  is  the  alpha- 
bet of  Morse's  telegraph  constructed  ? — 282.  Give  some  points  in  the  his- 
tory of  the  electric  telegraph.  What  is  the  distinction  between  invention 
and  discovery  ? 


APPENDIX. 


METRIC  SYSTEM  OF  WEIGHTS  AND  MEASURES. 

THE  metric  system  of  weights  and  measures  is  based 
upon  an  arbitrary  unit  called  the  metre.  The  simplicity  of 
the  system,  its  uniformity,  decimal  notation,  and  expressive 
nomenclature,  besides  the  fact  that  its  units  of  length,  vol- 
ume, and  weight  are  mutually  related  upon  scientific  prin- 
ciples, render  it  peculiarly  serviceable  in  all  experiments, 
calculations,  and  writings  of  a  scientific  character. 

The  immense  number  of  arbitrary  weights  and  measures, 
totally  devoid  of  uniformity,  which  existed  in  the  various 
countries  of  Europe  had  long  been  felt  as  exceedingly  in- 
convenient, laborious,  injurious  to  international  commerce, 
•and  as  "inexhaustible  fountains  of  diversity,  confusion,  and 
fraud,"  France,  feeling  the  great  evil,  took  advantage  of 
the  revolutionary  spirit  which  prevailed  at  the  close  of  the 
last  century,  and  set  herself  vigorously  to  work  to  over- 
come it. 

The  proposition  for  the  creation  of  a  metric  system  orig- 
inated in  1790  with  Prince  Talleyrand,  who  introduced 
into  the  National  Assembly  of  France  a  decree  providing 
for  a  commission  to  select  a  unit  of  measure  and  to  build 
up  a  complete  system.  Talleyrand  had  favored  the  adop- 


416          METRIC    SYSTEM    OF   WEIGHTS    AND    MEASURES. 

tion  for  a  linear  unit  of  the  length  of  a  pendulum  beating 
seconds  in  latitude  45°;  but,  for  reasons  upon  which  we 
cannot  dwell,  the  committee  of  the  Academy  of  Sciences 
decided  in  favor  of  deriving  the  unit  of  length  from  some 
one  of  the  natural  dimensions  of  the  earth,  and  recommend- 
ed as  the  standard  unit  of  linear  measure  one  ten  millionth 
of  the  quadrant  of  a  meridian. 

Delambre  and  Mechain,  two  distinguished  astronomers, 
were  charged  with  the  measurement  of  an  arc  of  the  merid- 
ian passing. through  Paris  and  extending  from  Dunkirk  to 
Barcelona — an  operation  of  immense  labor,  which  occupied 
seven  years. 

From  the_length  of  this  base  line,  which  was  determined 
with  the  greatest  possible  accura- 
cy, that  of  the  distance  from  the 
equator  to  the  pole  was  calculated, 
and  the  ten  millionth  part  of  this 
was  taken  as  the  standard  unit  of 
length :  this  length,  equal  to  three 
feet  three  inches  and  three  eighths 
nearly,  was  called  a  metre  from  the 
Greek  metron — "a  measure."  A 
bar  of  platinum  representing  this  length  was  constructed 
as  a  standard  and  deposited  in  the  Palace  of  the  Archives 
in  Paris. 

From  this  standard  the  measures  of  surface,  capacity, 
and  weight  are  derived  on  principles  explained  below. 
The  metric  system  was  declared  legal  in  France  in  1799, 
and  was  made  obligatory  in  1840.  Since  then  it  has  been 
adopted  by  most  of  the  countries  of  Europe,  and  is  in  par- 
tial use  by  every  nation  of  Christendom. 

Units  of  the  Metric  System. — -The  measures  of  surface, 
capacity,  and  weight  are  connected  with  the  unit  of  length, 
the  metre,  by  very  simple  relations. 


APPENDIX. 


417 


The  unit  of  length,  called  METRE,  is  one  ten-millionth  of  the  quadrant  of 

a  terrestrial  meridian. 

The  unit  of  surface,  called  ARE,  is  equal  to  a  square  whose  side  is  ten 

metres,  and  is  consequently  one  hun- 
dred square  metres. 

The  unit  of  volume,  called  LITRE,  is  the  volume  of  a  cube  whose  edge 

is  one  tenth  of  a  metre,  and  is  con- 
sequently one  thousand  cubic  centi- 
metres. 

The  unit  of  capacity,  called  STERE.  is  a  cubic  metre. 

The  unit  of  iveiyht,  called  GRAMME,  is  the  weight  of  a  cubic  centimetre  (one 

thousandth  part  of  a  litre)  of  distilled 
water  at  4°  C. 

The  subdivisions  and  multiples  of  these  measures  are 
decimal,  and  are  indicated  by  Latin  prefixes  for  the  former 
and  Greek  prefixes  for  the  latter.  Thus  twelve  words  suf- 
fice to  express  the  units,  multiples,  and  fractions  of  the 
whole  metric  system.  These  are: 

(  (1)  Metre,  from  the  Greek  metron,  signifying  a  measure. 


rf  \  (2)  Litre, 

" 

"      £i<ra, 

" 

pound. 

•5  J  (3)  Gramme, 

a 

"      gramma, 

H 

small  weight. 

P  )  (4)  Are, 

u 

Latin  area, 

(( 

surface. 

(  (5)  Stere, 

(1 

Greek  stereos. 

U 

solid. 

*  (  (6)  Milli, 
|  \  (7)  Centi, 

«« 
tt 

Latin  mille, 
centum, 

(4 
M 

one  thousand, 
one  hundred. 

I  '  (8)  Deci, 

II 

"      decem, 

" 

ten. 

.  (  (0)  Deka, 

M 

Greek  rfe&a, 

(4 

ten. 

!j(10)IIecto, 

" 

"      hekaton, 

« 

one  hundred. 

!)  (11)  Kilo, 

fl 

"      cAiVio*, 

(( 

one  thousand. 

a  ((12)Myria, 

l( 

"      myrias, 

t( 

ten  thousand. 

Most  of  these  words  are  already  in  use  in  the  English 
language :  thus  metre  occurs  in  thermometer,  metrology, 
etc.;  litre  in  litrameter;  gramme  in  telegram,  etc.;  are  in 
area;  stere  in  stereoscope;  'mitti  in  millennium  and  mill; 
centi  in  century  and  cent;  deci  in  decimal;  deka  in  decade; 
liecto  in  hecatomb ;  myria  in  myriad. 

S2 


418          METRIC    SYSTEM    OP   WEIGHTS    AND   MEASURES. 

The  manner  in  which  these  terms  are  applied  is  shown 
in  the  following  table,  which  comprises  the  whole  metric 
system-. 

Metric  System  of  Weights  and  Measures. 

MONEY. 

10  mills  make  one  cent. 
10  cents      "       "    dime. 
10  dimes    "       "    dollar. 
10  dollars  "       "    eagle. 

LENGTH. 

10  millimetres  make  one  centimetre. 

10  centimetres      "       "  decimetre. 

10  decimetres       "       "  metre. 

10  metres  "       "  dekametre. 

10  dekametres      "       "  hectometre. 

10  hectometres    "       "  kilometre. 

10  kilometres       "       '*  myriametre. 

WEIGHT. 

10  milligrammes  make  one  centigramme. 

10  centigrammes      "       "  decigramme. 

10  decigrammes       "       "  gramme. 

10  grammes  "       "  dekagramme. 

10  dekagrammes      "       "  hectogramme. 

10  hectogrammes     "       "  kilogramme. 

CAPACITY. 

10  millilitres  make  one  centilitre. 
10  centilitres      "       "    decilitre. 
10  decilitres       "       "    litre. 
10  litres  "       "    dekalitre. 

10  dekalitres      "       "    hectolitre. 

The  SQUARE  and  CUBIC  MEASURES  are  simply  the  squares 
and  cubes  of  the  measures  of  length.  The  square  deka- 
metre having  received  the  name  are,  and  the  cubic  metre 
the  name  stere,  as  before  stated ;  the  stere,  however,  is  not 
in  common  use,  its  place  being  taken  by  the  litre. 


APPENDIX.  419 

The  following  diagram*  explains  itself: 


-i 


Each  side  of  this  square  measures 

1  Decimetre,  or 
10  Centimetres,  or 
100  Millimetres,  or 
3.937  English  inches. 

A  litre  is  a  cubic  measure  of  1  decimetre  in  the  side,  or  a  cube 
each  side  of  which  has  the  dimensions  of  this  figure. 

When  full  of  water  at  4°  C.  a  litre  weighs  exactly  1  kilogramme, 
or  1000  grammes,  and  is  equivalent  to  1000  cubic  centimetres,  or 
to  61.024  cubic  inches,  English. 

A  gramme  is  the  weight  of  a  centimetre  cube  of  distilled  water ; 
at  4°  C.  it  weighs  15.432  grains. 


Centim- 
etre. 

10 


-4  inches. 


Comparison  of  English  and  Metric  Measures. — The  metre 
corresponds  to  the  English  yard,  and  approximates  to  three 
feet  three  inches  and  three  eighths  of  an  inch.  Five  metres 
are  nearly  one  rod.  The  kilometre,  which  is  used  for  meas- 
uring distances  as  we  use  the  English  mile,  is  a  little  less 

*  From  "Introduction  to  the  Study  of  Inorganic  Chemistry,"  by  William 
Allen  Miller,  M.D. 


420 


METRIC    SYSTEM    OF    WEIGHTS    AND    MEASURES. 


than  200  rods,  or  -|  of  a  mile.  The  litre  is  a  little  more 
than  a  wine  quart.  The  kilogramme  is  the  unit  by  which 
articles  sold  by  weight  are  measured,  and  is  about  2^ 
pounds.  The  five -cent  nickel  coin  of  the  United  States 
Mint  weighs  5  grammes,  and  is  20  millimetres  in  diameter. 
It  is  often  desirable  to  know  the  exact  value  of  the  units, 
as  given  below : 

1  Metre =    39.37079  inches. 

1  Kilometre =      0.62138  miles. 

1  Are =1 19.60332  square  yards. 


1  Hectare 

1  Litre 

1     "    

1  Gramme 

1  Kilogramme, 


2.47114  acres. 
0.26418635  gallons,  or 
1.0567454  quarts. 
15.43234874  grains. 
2.20462125  Ibs.  avoirdupois. 


The  following  diagram  shows  the  relations  of  the  English 


and  metric  measures  of  length : 


Indies. 


1 

1 

1 

2 

3 

4 

One    Decimetre. 


1O    Centimetres. 


10 


10O    Millimetres. 


The  following  tables  give  the  metric  measures  legalized 
in  the  United  States,  with  the  abbreviations  and  their 
equivalents  now  in  use : 


APPENDIX. 


421 


MEASURES  OF  LENGTH. 


Metric  Denominations. 

Abbrevia- 
tions. 

Values. 

Equivalents  Legalized  by 
Congress  in  Denomina- 
tions now  in  Use. 

Myriametre  

Mm. 

10,000         m. 

6.2137  miles. 

Kilometre    • 

Km. 

1,000         m. 

0.62137    " 

Hectometre        . 

Hm. 

100         m. 

or,  3280  ft.  10  in. 
328  ft.    1  in. 

Dekametre  

Dm. 

10-         m. 

393.7          " 

Metre  

m. 

1          m. 

39.37 

Decimetre      

dm. 

.1      m. 

3.937      " 

Centimetre  

cm. 

.01     m. 

0.3937    " 

Millimetre  

mm. 

.001  m. 

0.03937  " 

MEASURES  OF  SURFACE. 


Metric  Denominations. 

Abbrevia- 
tions. 

Values. 

Equivalents  Legalized  by 
Congress  in  Denomina- 
tions now  in  Use. 

Hectare  

Ha. 

10,000  sq.  m. 

2.471  acres. 

Are 

a 

100  sq.  m. 

119.6  sq.  yards. 

Centare  

ca. 

1  sq.  m. 

1  550  sq.  inches. 

MEASURES  OF  CAPACITY. 


Metric  Names. 

Abbrevia- 
tions. 

No.  of 

Litres. 

Dry  Measure. 

Liquid  or  Wine 
Measure. 

Kilolitre,  or  Stere. 
Hectolitre.      . 

Kl.,st. 
HI. 

1000 
100 

1.308  cu.  yds. 
2bu.  3.35pks. 

264.17     gals. 
26.417      " 

Dekalitre  

Dl. 

10 

9.08  qts. 

2.6417    " 

Litre                . 

1 

1 

0  098  qt. 

1.0567  qts. 

Decilitre  

dl. 

.1 

6.1022  cu.  in. 

0.845  gill. 

Centilitre 

cl 

01 

0  6102  "    " 

0.338fld.oz. 

Millilitre  

ml 

.001 

0.061     "    " 

0.27  fld.  dr. 

WEIGHTS. 


Metric  Denominations  and  Values. 

Weight  of  what 
Quantity  of  Wa- 
ter at  Maximum. 

Equivalent  in 
Denominations 
now  in  Use. 

Avoirdupois  W'glit. 

Names. 

Abbrevia- 
tions. 

No.  of 
Grammes. 

Millier,  or  Tonneau. 
Quintal  

M.,orT. 
Q. 

Mg. 
Kg. 
Hg. 
Dg. 
g- 
dg. 
eg. 
mg. 

1,000,000 
100,000 
10,000 
1,000 
100 
10 

1 
.1 

.01 
.001 

1  cu.  metre. 
1  hectolitre. 
10  litres. 
1  litre. 
1  decilitre. 
10    cu.  centim. 
1      a.       a 

.1  "       " 
10  cu.  millim. 
1    "        " 

2204.6      Ibs. 
220.46      " 
22.046    " 
2.2046  " 
3.5274oz. 
0.3527  " 
15.432  grs. 
1.5432  " 
0.1543  " 
0.0154  " 

Myriagramme  
Kilogramme,  or  Kilo. 
Hectogramme  
Dekagramme     .... 

Gramme. 

Decigramme  

Centigramme 

Milligramme  

INDEX. 


[The  numbers  refer  to  the  pages.] 


A. 


Absolute  and  relative  motion,  100. 

Accelerated  force  illustrated,  106. 

Accelerated  motion,  98. 

Acoustics  defined,  235. 

Actinism,  347. 

Activity  of  electricity,  357. 

Adhesion,  67. 

Adhesion,  cohesion,  and  gravitation 

compared,  76. 
Advantages  in  the  use  of  machinery, 

155. 
Aeriform  substances,  expansion  in, 

264. 

Air  a  non-conductor  of  heat,  287. 
Air  as  a  non-conductor  in  walls  of 

houses,  288. 

Air  attracted  by  the  earth,  209. 
Air,  compressibility  of,  210. 
Air,  currents  in,  caused  by  heat,  2G5. 
Air,  density  of,  dependent  upon  press- 
ure, 217. 

Air,  elasticity  of,  219. 
Air-guns,  221. 
Air  in  pores  of  bodies,  219. 
Air,  liquefaction  of,  220. 
Air  material  and  has  weight,  208. 
Air,  pressure  of,  and  boiling-point 

of  liquids,  226. 

Air,  pressure  of,  on  liquids,  221. 
Air-pump,  description  of  the,  213. 
Air-pump,    experiments    with    the, 

214. 


Air,  thickness  of  the,  210. 

Air,  weight  of,  209. 

Alum,  crystalline  form  of,  63. 

Amalgamation  of  zinc,  380. 

Amber,  properties  of,  351. 

Amorphous  bodies,  66. 

Aneroid  barometer,  225. 

Animal,  man  a  tool-making,  157. 

Animals,  specific  gravity  of,  184. 

Annealing,  44. 

Anvil  trick  explained,  110. 

Apparatus  for  electrical  experiments, 

351. 

Archimedes  and  the  crown,  188. 
Archimedes's    discovery   of  specific 

gravity,  184. 
Archimedes's  lever,  138. 
Archimedes's  screw,  204. 
Are,  value  of  the,  418. 
Aristotle's  definition  of  man,  157. 
Artesian  wells,  167. 
Atmosphere,  moisture  in,  271. 
Atmosphere,  pressure  of  the,  212. 
Atmosphere,  thickness  of  the,  210. 
Atmospheric    pressure,   amount    of, 

223. 

Atoms,  25. 

Atoms,  weight  of  hydrogen,  26. 
Attraction   an  effect  of  electricity, 

353. 

Attraction,  capillary,  72. 
Attraction,  chemical,  78. 
Attraction,  names  given  to  different 

modes  of,  52. 


424 


INDEX. 


Attraction  of  the  earth  exerted  on 
liquids,  172. 

Attraction,  opposition  between  the 
modes  of,  73. 

Attraction,  proportion  of  the  mutual 
motions  of,  53. 

Attraction  towards  the  earth's  cen- 
tre, 54. 

Attraction,  variety  in  the  results  of, 
78. 

B. 

Balance,  example  of  lever,  135. 

Balloons,  265. 

Balloon,  hydrostatic,  218. 

Barker's  Mill,  207. 

Barometer,  aneroid,  225. 

Barometer  described,  223. 

Barometer,    use    of,    in    measuring 

heights,  225. 

Battery,  Bunsen's  galvanic,  385. 
Battery,  Daniell's  galvanic,  384:. 
Battery,  electrical,  370. 
Battery,  galvanic,  383. 
Battery,  Grove's  galvanic,  385. 
Bellows,  hydrostatic,  178. 
Bends  in  rivers,  124. 
Boats  and  life-boats,  184. 
Bodies,  unstable,  87. 
Bodies,  why  they  fall  in  air,  210. 
Boiling-point,  air's  pressure  and  the, 

226. 

Boiling-points  of  liquids,  276. 
Bore,  meaning  of,  201. 
Bramah's  hydrostatic  press,  179. 
Brewery  vats,  171. 
Bridge  of  nails,  407. 
Brittleness,  43. 
Bud  of  plants  in  winter,  291. 
Bullets,  cohesion  of,  59. 
Bunsen's  galvanic  battery,  385. 

C. 

Camera  lucida,  332. 
Camera  obscura,  331. 


Canals,  locks  of,  163. 

Capacity  for  heat,  300. 

Capillary  attraction,  72. 

Capstan,  144. 

Case  in  court  illustrating  inertia, 
120. 

Cask,  bursting  of,  by  a  column  of 
water,  176. 

Centigrade  thermometer,  263. 

Centre  of  gravity  and  attitudes,  90. 

Centre  of  gravity,  determination  of, 
81. 

Centre  of  gravity  illustrated,  79. 

Centre  of  gravity  in  floating  bodies, 
91. 

Centre  of  gravity,  location  of,  in  hol- 
low bodies,  82. 

Centre  of  gravity,  motions  of,  in 
walking,  89. 

Centre  of  gravity  seeks  the  lowest 
point,  82. 

Centre  of  gravity,  support  of,  in  ani- 
mals, 88. 

Centrifugal  force,  121. 

Centrifugal  force,  application  of,  in 
the  arts,  1 24. 

Centrifugal  force  and  shape  of  the 
earth,  127. 

Centripetal  force,  121. 

Chemical  action  a  source  of  heat, 
255. 

Chemical  attraction,  78. 

Chemical  effects  of  galvanism,  390. 

Chimneys,  draught  of,  266. 

Circle,  compound  voltaic,  383. 

Circle,  simple  voltaic,  379. 

Clepsydra,  198. 

Clothing,  290. 

Clouds  and  latent  heat,  302. 

Clouds,  formation  of,  271. 

Clouds,  shapes  of,  272. 

Cocoons,  291. 

Cohesion,  58. 

Cohesion  in  liquids,  59. 

Cold  produced  by  evaporation,  303. 


INDEX. 


425 


Colors,  elementary  names  of,  342. 
Colors  in  dew-drops  and  ice-crystals, 

345. 

Colors  of  objects,  342. 
Communication  of  motion  in  elastic 

bodies,  131. 

Compass,  points  of  the,  401. 
Compound  motion,  100. 
Compressibility,  41. 
Conduction  of  heat,  282. 
Conductors  and  non-conductors  of 

heat,  283. 

Conductors  of  electricity,  358. 
Convection,  illustrated,  261. 
Convection  of  heat,  281. 
Count  Rumford's  discoveries,  253. 
Course  of  bodies  thrown  into  the  air, 

112. 

Crown  of  cups,  382. 
Crystallization,  63. 
Cup  of  Tantalus,  230. 
Cylindrical  electrical  machine,  364. 

I). 

Bams  and  sluice-gates,  170. 

Daniell's  galvanic  battery,  384. 

Davy,  Sir  Humphry,  experiment  of, 
388. 

Davy's  safety-lamp,  284. 

Dawn  and  twilight,  324. 

Decomposition  of  water  by  galvan- 
ism, 392. 

Density,  40. 

Density,  relation  of,  to  conduction, 
286." 

Dew,  formation  of,  295. 

Dew-point,  296. 

Diamagnetism,  405. 

Diatonic  scale,  248. 

Digester,  277. 

Dip  of  magnetic  needle,  402. 

Discharging  rod,  369. 

Diving-bell,  31. 

Divisibility  of  matter,  35. 

Double  rainbow,  formation  of,  345. 


Double  windows,  uses  of,  288. 
Drops,  globular  shape  of,  (!0. 
Drops,  size  of,  influenced  by  gravita- 
tion, 74. 

Drowning,  avoidable  causes  of,  185. 
Ductility,  48. 


Earth  a  great  magnet,  403. 
Earth  a  magnetizer,  404. 
Earth  a  source  of  heat,  255. 
Earth  and  the  atmosphere,  113. 
Earth,  shape  of,  influenced  by  cen- 
trifugal force,  1 27. 
Earth,  spherical  form  of,  62. 
Ear-trumpet,  244. 
Echoes,  cause  of,  242. 
Effects  of  liquid  pressure,  176. 
Elasticity,  42. 
Elasticity  of  the  air,  219. 
Electric  telegraph,  408. 
Electrical  battery,  370. 
Electrical  induction,  361. 
Electrical  machine,  experiments  with, 

365. 

Electrical  machine,  operation  of,  363. 
Electricity,  activity  of,  357. 
Electricity  always   on   the   surface, 

359. 

Electricity,  applications  of,  412. 
Electricity,  characteristics  of  voltaic, 

386. 
Electricity  discharged  from  points, 

366. 

Electricity,  distribution  of,  360. 
Electricity,  dynamical,  378. 
Electricity,  effects  of,  351. 
Electricity,    frictional    and    voltaic, 

compared,  386. 

Electricity,  heat  produced  by,  371. 
Electricity,  light  of,  370. 
Electricity,  mechanical  effects  of,  371. 
Electricity,    positive    and    negative, 

356. 
Electricity,  theories  of,  355. 


428 


INDEX. 


Electricity,  voltaic,  378. 

Electrics  and  non-electrics,  359. 

Electrified  bodies,  352. 

Electrolysis,  392. 

Electro-magnetism,  405. 

Electro-metallurgy,  393. 

Electrometer,  363. 

Electro-positive  and  electro-negative, 
381. 

Electroscope,  3G2. 

Electrotyping,  393. 

Emission  theory  of  light,  312. 

English  and  metric  measures  com- 
pared, 419. 

Equestrian  feats  explained,  120, 123. 

Evaporation,  270. 

Expansibility,  41. 

Expansion,  different  rates  of,  260. 

Expansion  in  aeriform  bodies,  264. 

Expansion  of  solids  by  heat,  258. 

Experiments  illustrating  centre  of 
gravity,  85. 

Extension,  29. 

Eye,  description  of  the,  332. 

Eyes,  near-sighted  and  far-sighted, 
335. 

F. 

Fahrenheit's  thermometer,  262. 

Fall  of  bodies  in  vacuo,  109. 

Falling  bodies  really  projected,  116. 

Faraday's  experiment,  360. 

Figure,  29. 

Fire-engine,  action  of,  232. 

Fishes,  shape  of,  203. 

Flame  and  wire  gauze,  285. 

Flexibility,  43. 

Fluids,  elastic  and  non-elastic,  19. 

Fluids,  lateral  pressure  in,  171. 

Force,  accelerated,  106. 

Force,  centrifugal   and    centripetal, 

121. 
Force,  centrifugal,  and  shape  of  the 

earth,  127. 
Force  defined,  94. 


Force,   illustrations    of   centrifugal, 

122. 

Force-pump,  219. 
Force,  retarded,  110. 
Force,  relation  of,  to  velocity,  105. 
Forcing-pumps,  232. 
Franklin's  discovery — lightning,  372. 
Franklin's  theory  of  electricity,  355. 
Freezing  in  the  midst  of  boiling,  303. 
Freezing  mixtures,  302. 
Freezing  of  water  explained,  306. 
Friction,  advantage  taken  of,  154. 
Friction  an  obstacle  to  motion,  153. 
Friction  a  source  of  heat,  256. 
Friction,  expedients  for  diminishing, 

154. 

Friction  in  streams,  199. 
Friction  of  liquids  in  pipes,  199. 
Frog,  experiments  in  galvanism  with, 

376. 

Frost  and  snow,  64. 
Fulcrum  defined,  134. 
Fur,  hair,  and  feathers,  289. 
Fusee  of  watches,  144. 

G. 

Galileo  and  the  pendulum,  151. 
Galvani's  discovery,  375. 
Galvanism  and  the  decomposition  of 

water,  391. 
Galvanism,  burning   of  metals   by, 

388. 

Galvanism,  characteristics  of,  386. 
Galvanism,  chemical  effects  of,  390. 
Galvanism,  discovery  of,  375. 
Galvanism,  heating  effects  of.  387. 
Galvanism,  physiological  effects  of, 

395. 

Galvanism,  polarity  in,  380. 
Galvanism  produces  light,  389. 
Galvanism,  ways  of  producing,  382. 
Gamut,  musical,  248. 
Ganges,  bore  in,  201. 
Ganges,  course  of,  1 62. 
Garnet,  crystalline  form  of,  64. 


INDEX. 


427 


Gases  and  liquids,  similarity  between, 
211. 

Gases,  characteristics  of,  20. 

Gases,  compressibility  of,  41. 

Gases,  relation  of  bulk  to  the  resist- 
ance of,  202. 

Gases,  specific  gravity  of,  194. 

Gauze,  wire,  and  flame,  285. 

Geneva,  Lake  of,  163. 

Gideon's  Fleece,  296. 

Gilding  and  silver-plating,  394. 

Glass,  annealing  of,  44. 

Glass-blowing,  125. 

Glass,  elasticity  of,  43. 

Glass,  tempered,  45. 

Gold,  divisibility  of,  36. 

Gold,  malleability  of,  48. 

Gramme,  weight  of  the,-  419. 

Gravitation,  52. 

Gravity,  action  of,  on  solids  in  a  liq- 
uid, 182. 

Gravity  a  uniformly  accelerating 
force,  108. 

Gravity,  centre  of,  79. 

Gravity,  specific,  181. 

Grove's  galvanic  battery,  385. 

H. 

Hail,  snow,  and  rain,  275. 

Hardness,  45. 

Hardness,  scale  of,  46. 

Harmony,  247. 

Heat  and  cold  relative  terms,  252. 

Heat  and  light,  analogy   between, 

314. 

Heat,  capacity  for,  300. 
Heat,  communication  of,  281. 
Heat,  conduction  of,  and  sensation, 

292. 
Heat,  degree  of,  endurable  by  man, 

305. 

Heat,  latent,  297. 
Heat,  latent,  illustrated,  299. 
Heat,  light,  and  chemical  rays,  346. 
Heat,  nature  of,  253. 


Heat  produced  by  electricity,  371. 

Heat,  radiation  of,  293. 

Heat,  recent  theory  of  latent,  298. 

Heat,  reflection  of,  294. 

Heat,  relation  of,  to  forms  of  matter, 
21. 

Heat,  sources  of,  254. 

Heat,  water  an  exception  to  expan- 
sion by,  307. 

Heavenly  bodies,  spherical  form  of, 
62. 

Helix,  description  of  a,  406. 

Historical  sketch  of  the  telegraph, 
411. 

Honey-combs  and  centrifugal  force, 
126. 

Horizontal  projectile,  path  of,  114. 

Horseshoe  magnets,  401. 

Human  bodies,  specific  gravity  of, 
185. 

Human  voice,  247. 

Huyghens's  theory  of  light,  313. 

Hydraulics,  definition  of,  196. 

Hydrometer,  use  of,  192. 

Hydrostatic  balloon,  218. 

Hydrostatic  bellows,  178. 

Hydrostatic  paradox,  178. 

Hydrostatic  press,  179. 

Hydrostatics,  objects  of,  159. 

I. 

Ice,  force  of  expansion  in,  308. 

Ice,  formation  of,  305. 

Icebergs  and  centre  of  gravity,  92. 

Illustrations  of  compound  motion, 
101. 

Illustrations  of  the  third  law  of  mo- 
tion, 130. 

Images  in  the  eye  inverted,  335. 

Impenetrability,  30. 

Inclined  plane,  147. 

Indestructibility,  32. 

Induction,  electrical,  361. 

Induction,  magnetism  by,  398. 

Inertia,  33. 


428 


INDEX. 


Inertia,  experiments  in,  34. 

Inertia  shown  in  the  communication 

of  motion,  118. 
Inertia  shown  in  the  disposition  of 

motion  to  continue,  119. 
Insulating  stool,  305. 
Insulators,  358. 
Intermitting  springs,  230. 

J. 
Jar,  the  Leyden,  367. 

K. 

Kaleidoscope  explained,  321. 
Knives,  chisels,  etc.,  wedges  in  prin- 
ciple, H8. 

L. 

La  Bastie  glass,  45. 

Lactometer,  use  of,  193. 

Land  breezes,  267. 

Lantern,  magic,  330. 

Latent  heat,  297. 

Latent  heat,  relation  of,  to  density, 

301. 

Lateral  pressure  of  liquids,  1 72. 
Laws  of  motion,  Newton's,  117. 
Lenses  described,  328. 
Lever,  no  power  gained  by,  137. 
Lever  of  the  first  kind,  134. 
Lever  of  the  second  kind,  139. 
Levers,  compound,  142. 
Lewis,  description  of  a,  157. 
Leyden-jar,  construction  of,  367. 
Leyden-jar,  discharge  of,  369. 
Life- boats,  184. 
Light,  analogy  with  sound  and  heat, 

346. 
Light,  course  of,  through  a  prism, 

339. 

Light,  decomposition  of,  338. 
Light,  dispersion  of,  340. 
Light,  intensity  of,  316. 
Light,  nature  of,  312. 
Light  of  electricity,  370. 


Light,  production  of,  by  galvanism, 
389. 

Light,  radiation  of,  316. 

Light,  recomposition  of,  340. 

Light,  reflection  of,  319. 

Light,  refraction  of,  323. 

Light,  sources  of,  314. 

Light,  velocity  of,  318. 

Lightning  identical  with  electricity, 
372. 

Lightning-rods,  373. 

Liquefaction,  270. 

Liquid,  globular  shape  of  drops  of, 
60. 

Liquid  pressure  at  great  depths,  1 70. 

Liquid  pressure  equal  in  all  direc- 
tions, 173. 

Liquid  pressure,  illustrations  of,  1 75. 

Liquids,  action  of  gravity  on  solids 
in,  182. 

Liquids,  characteristics  of,  20. 

Liquids,  cohesion  in,  50. 

Liquids,  compressibility  of,  41. 

Liquids,  conduction  in,  287. 

Liquids,  expansion  of,  260. 

Liquids,  experiments  showing  up- 
ward pressure  in,  175. 

Liquids,  flow  of,  196. 

Liquids,  flow  of  through  tubes,  198. 

Liquids,  friction  of,  in  pipes,  199. 

Liquids,  how  to  determine  specific 
gravity  of,  191. 

Liquids,  influence  of  shape  on  resist- 
ance of,  203. 

Liquids,  level  surface  of,  159. 

Liquids,  mobility  of,  60. 

Liquids,  pressure  of,  168. 

Liquids,  pressure  of  air  on,  221. 

Liquids,  relation  of  bulk  to  the  re- 
sistance of,  202. 

Liquids,  rise  of,  in  tubes,  70. 

Liquids,  spherical  form  in  different, 
61. 

Liquids,  surface  of,  not  horizontal, 
160. 


INDEX. 


429 


Liquids,  tendency  of,  to  equilibrium, 

164. 

Litre,  value  of  the,  417. 
Loadstone,  397. 
Locks  of  canals,  163. 
Loggan  stones,  83. 

M. 

Machine,  electrical,  363. 

Machinery,  real  advantages  of,  155. 

Machinery,  repairing  of,  by  heating, 
258. 

Machines  for  raising  water,  204. 

Machines  not  sources  of  power,  133. 

Magdeburg  hemispheres,  215. 

Magic  lantern,  330. 

Magnet,  polarity  of  the,  398. 

Magnet,  the  earth  a  great,  403. 

Magnetic  curves,  400. 

Magnetic  needle,  401. 

Magnetism  by  induction,  398. 

Magnetism,  theory  of,  404. 

Magnetization  by  the  earth,  404. 

Magnets,  artificial,  400. 

Magnets,  horseshoe,  401. 

Magnets,  natural,  397. 

Malleability,  48. 

Man  a  tool-making  animal,  157. 

Marriotte's  law,  220. 

Matter  acted  upon  by  forces,  26. 

Matter  attracts  matter,  51. 

Matter,  compressibility  of,  41. 

Matter,  constitution  of,  23. 

Matter,  divisibility  of,  35. 

Matter,  effect  of,  on  the  senses,  18. 

Matter,  expansibility  of,  41. 

Matter,  forms  of,  1 9. 

Matter,  inertia  of,  33. 

Matter,  in  motion,  94. 

Matter,  motion,  and  force,  93. 

Matter,  porosity  of,  39. 

Matter,  properties  of,  28. 

Matter,  nature  of,  22. 

Matter,  usefulness  of  variety  in  prop- 
erties of,  49. 


Measures  and  weights,  metric  system 
of,  415. 

Mechanical  action  a  source  of  heat, 
256. 

Mercury,  non-adhesion  of,  68. 

Mercury  shower,  216. 

Metals,  burning  of,  by  electricity,  388. 

Metals,  order  of  oxidability  of,  386. 

Metre,  determination  of  length  of 
the,  416. 

Metric  system  of  weights  and  meas- 
ures, 415. 

Microscope  and  divisibility  of  matter, 
37. 

Microscopes  and  telescopes,  329. 

Mirages  explained,  325. 

Mirrors,  curved,  322. 

Mirrors,  reflection  in,  320. 

Mohammed's  coffin  realized,  407. 

Molecules,  24. 

Molecules,  cohesion  between  like,  59. 

Momentum  defined,  103. 

Momentum  estimated,  104. 

Momentum  illustrated.  105. 

Morse's  credit  as  inventor  of  the  tel- 
egraph, 412. 

Morse's  telegraphic  alphabet,  411. 

Morse's  telegraphic  machine,  409. 

Motion,  absolute  and  relative,  100. 

Motion,  accelerated,  retarded,  varia- 
ble, 98. 

Motion  and  rest  relative  terms,  99. 

Motion,  attempt  at  perpetual,  166. 

Motion,  causes  of,  95. 

Motion,  communication  of,  in  elastic 
bodies,  181. 

Motion,  compound,  100. 

Motion  defined,  94. 

Motion,  friction  an  obstacle  to,  153. 

Motion,  heat  a  mode  of,  253. 

Motion  in  orbits,  117. 

Motion,  Newton's  laws  of,  117. 

Motion,  perpetual,  129. 

Motion,  second  law  of,  129. 

Motion,  third  law  of,  129. 


430 


INDEX. 


Motion,  uniformity  of,  128. 
Motion,  universal,  95. 
Motion,  varieties  of,  96. 
Musical  notes  and  noises,  245. 
Musical  notes,  quality  of,  246. 

N. 

Natural  philosophy,  definition  of,  1 8. 
Needle,  declination  and  dip  of  the, 

402. 

Needle,  magnetic,  401. 
Negative  and  positive  electricity,  356. 
New  Haven,  mirage  visible  at,  326. 
Newton's  laws  of  motion,  117. 
Newton's    recomposition    of   white 

light,  341. 
Nutcrackers  examples  of  lever,  140. 

O. 

Oersted's   discovery   in   magnetism, 

406. 

Opaque  and  transparent  bodies,  315. 
Order  in  nature,  65. 
Otto  von  Guericke,  216,  364. 
Overshot  water-wheel,  206. 

P. 

Papin's  digester,  277. 

Parabolic  curve  of  projectiles,  113. 

Paradox,  hydrostatic,  178. 

Path  of  projectiles,  113. 

Pendulum,  description  of,  151. 

Pendulum,  gridiron,  260. 

Pendulum,  operation  of,  152. 

Pendulum,  vibrations  of,  152. 

Perpetual  motion,  attempted,  166. 

Photometers,  318. 

Physiological  effects    of  galvanism, 

395. 

Pile,  Volta's  electric,  377. 
Pisa,  pendulum  in  cathedral  of,  152. 
Piston  of  steam-engine,  motion  of, 

279. 

Plant-buds  in  winter,  291. 
Plants  protected  by  snow,  291. 


Platform  scales,  142. 

Pneumatic  trough,  description  of, 
222. 

Pneumatics,  objects  of,  208. 

Polarity  in  galvanism,  380. 

Polarity  of  the  magnet,  398. 

Pop-guns,  221. 

Porosity,  39. 

Positive  and  negative  electricity,  356. 

Press,  hydrostatic,  179. 

Pressure  in  fluids,  lateral,  171. 

Pressure  in  liquids,  equal  in  all  di- 
rections, 1 73. 

Pressure  of  liquids,  168. 

Prince  Rupert's  Drops,  45. 

Prism,  glass,  339. 

Projectiles,  path  of,  113. 

Properties  of  matter  classified,  29. 

Pulleys,  complex  arrangements  of, 
146. 

Pulleys,  fixed  and  movable,  145. 

Pumps,  forcing,  232. 

Pumps,  operation  of,  231. 

Pyramids,  Egyptian,  construction  of, 
148. 

Q. 

Quality  of  musical  sounds,  246. 
Quartz,  crystalline  form  of,  64. 

R. 

Radiation  of  heat,  294. 
Radiometer,  Crookes's,  348. 
Rain,  snow,  and  hail,  275. 
Rainbow,  circular,  346. 
Rainbow,  explanation  of  the  forma- 
tion of,  343. 

Rainbows,  when  seen,  344. 
Rarity,  40. 
Rays,  chemical,  347. 
Rays  of  heat,  light,  and  actinism,  346. 
Reaumur's  thermometer,  263. 
Recomposition  of  light,  340. 
Reflection  of  heat,  294. 
Reflection  of  light,  319. 


INDEX. 


431 


Keflection  of  sound,  242. 

Refraction  of  light,  323. 

Repulsion  an  effect  of  electricity, 
353. 

Resistance  of  liquids  to  solids,  202. 

Retarded  force,  110. 

Retarded  motion,  98. 

Richman,  death  of  Professor,  from 
lightning  stroke,  372. 

Rivers,  flow  of,  161. 

Rivers,  how  some  have  been  made, 
162. 

Roemer's  determination  of  the  veloci- 
ty of  light,  319. 

Rope-dancing  and  centre  of  gravity, 
91. 

S. 

Safety-lamp,  Davy's,  284. 
Salt,  crystalline  form  of,  63. 
Scale  of  temperature,  309. 
Scales  and  centre  of  gravity,  81. 
Scales    of    different    thermometers, 

264:. 

Science,  definition  of,  17. 
Science,  subdivisions  of,  18. 
Scissors  example  of  levers,  137. 
Screw,  Archimedes's,  204. 
Screw,  a  simple  machine,  149. 
Screw-presses,  150. 
Screws,  estimation  of  power  of,  149. 
Sea  breezes,  268. 
Seesaw,  example  of  lever,  1 38. 
Shadows,  formation  of,  316. 
Sheathing  of  vessels,  380. 
Ships  lost  by  changing  centre  of  grav- 
ity, 92. 

Shot,  manufacture  of,  62. 
Silver-plating,  394. 
Simple  machines,  133. 
Siphon,  construction  of,  227. 
Siphon,  uses  of,  229. 
Sluice-gates  and  dams,  170. 
Smee's  battery,  393. 
Snow  a  protection  to  plants,  291. 


Snow,  crystalline  forms  of,  65. 
Snow,  rain,  and  hail,  275. 
Solid  bodies,  size  of,  limited  by  gravi- 
tation, 75. 

Solids  and  liquids,  adhesion  of,  68. 
Solids,  characteristics  of,  19. 
Solids,  expansion  of,  257. 
Solution,  21,  32. 

Sonorous  bodies,  vibrations  of,  235. 
Sound    and   hearing,   mysteries    of, 

249. 
Sound  caused  by  the  resistance  of 

the  air,  239. 

Sound,  concentration  of,  244. 
Sound,  definition  of,  235. 
Sound,  how   the    sensation    is   pro- 
duced, 237. 

Sound,  intensity  of,  245. 
Sound,  loudness  of,  241. 
Sound,  measurement  of  distances  by, 

240. 

Sound  not  transmitted  in  vacuo,  238. 
Sound,  reflection  of,  241. 

Sound  transmitted  through  various 
substances,  238. 

Sound,  velocity  of,  239. 

Sounds,  musical,  245. 

Spaces,  filling  of,  by  liquids  and  gas- 
es, 21. 

Speaking-trumpet,  244. 

Specific  gravity,  nature  of,  181. 

Specific  gravity  of  gases,  194. 

Specific  gravity  of  liquids,  determina- 
tion of,  191. 

Specific  gravity  of  solids,  how  to  'de- 
termine, 187. 

Specific  gravity,  tables  of,  194. 

Specific  properties  of  matter,  29. 

Spectrum,  solar,  339. 

Spirit-level,  description  of,  161. 

Springs,  intermitting,  230. 

Springs  and  wells,  167. 

Stability  of  bodies,  86. 

Steam-engine,  operation  of,  278. 

Steam-governor,  1 26. 


432 


INDEX. 


Steam,  relative  volume  of  water  and, 

40. 

Steel,  flexible  and  brittle,  44. 
Steelyards  examples  of  levers,  136. 
Stere,  value  of  the,  418. 
Stereoscope,  336. 

Structure  of  animals  and  levers,  141. 
Sucker,  operation  of,  216. 
Suez,  Isthmus  of,  canal  through  the, 

164. 
Sugar-refining  and  centrifugal  force, 

125. 

Sulzer's  observations,  375. 
Sun  a  source  of  heat,  254. 

T. 

Table  for  comparing  Fahrenheit  and 

Centigrade  thermometers,  264. 
Table  of  comparative  velocities,  97. 
Table  of  high  and  low  temperatures, 

310. 

Table  of  metric  measures,  421. 
Table  of  the  metric  system  of  weights 

and  measures,  418. 
Table  showing  comparative  tenacity 

of  materials,  47. 

Table  showing  scale  of  hardness,  46. 
Tables  of  specific  gravity,  194. 
Tantalus,  cup  of,  230. 
Tenacity,  47. 

Tenacity,  applications  of,  48. 
Telegraph,  electric,  408. 
Telegraph,  invention  of  the,  411. 
Telegraphing  by  Morse's  machine, 

410. 

Thaumatrope,  337. 
Thermometer,  Fahrenheit's,  262. 
Thermometers,  261. 
Thermometric  scales,  263. 
Tides,  cause  of,  20 1 . 
Time,  measurement  of,  151. 
Toggle-joint  a  simple  machine,  150. 
Tongs  examples  of  lever,  141. 
Tools,  advantages  of,  156. 
Torricellian  vacuum,  221. 


Tower  of  Pisa,  87. 

Toys  illustrating  centre  of  gravity, 

83. 

Tubes,  flow  of  liquids  through,  198. 
Turbine  water-wheel,  206. 

U. 

Undershot  water-wheel,  205. 
Undulatory  theory  of  light,  313. 
Uniform  motion  and  clocks,  98. 
Unison  in  music,  249. 
Units  of  the  metric  system,  416. 
Universal  properties  of  matter,  29. 
Unstable  bodies,  87. 
Up  and  down,  terms  explained,  55. 
Upward  pressure  in  liquids  as  the 
depth,  174. 

V. 

Vacuum,  223. 

Vacuum,    sound     not     transmitted 

through  a,  238. 
Vaporization,  276. 
Velocity,,  comparative,  of  bodies,  etc., 

96. 

Velocity  of  light,  318. 
Vessels,  sheathing  of,  380. 
Vibrations  of  sonorous  bodies,  236. 
Vision,  distinct,  334. 
Vision,  single,  336. 
Visual  angle,  327. 
Voice,  human,  247. 
Voltaic  circle,  378. 
Volta's  crown  of  cups,  382. 
Volta's  pile,  377. 

W. 

Walls  of  a  building  straightened  by 
heating  bars,  259. 

Water  and  ice,  relative  density  of, 
306. 

Water-clocks,  198. 

Water,  decomposition  of,  by  galvan- 
ism, 391. 

Water  frozen  by  evaporation,  304. 


INDEX. 


433 


Water-hammer,  109. 
Water-wheels,  205. 
Wave-motion  explained,  313. 
Waves,  formation  of,  200. 
Waves,  height  of,  200. 
Wedge,  power  and  use  of,  148. 
Weighing  air,  209. 
Weight  defined,  56. 
Weight  variable,  57. 
Weight  varies  with  distance,  57. 
Weights  and  measures,  metric  sys- 
tem of,  415. 


Welding-iron,  49. 

Wells,  artesian,  and  springs,  1G7. 

Wheel  and  axle,  143. 

Whispering-galleries,  243. 

Windlass,  143. 

Winds,  267. 

Winds  affected  by  rotation   of  the 

earth,  269. 
Wollaston's  wire,  48. 


Zinc,  amalgamation  of,  380. 


THE    END, 


•  - 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


